The delegation met with top military leaders and troops at Bagram Air Field, the main U.S. military base in the country, according to a U.S. military statement.
The Senator also stopped in Jalalabad in eastern Afghanistan to meet with local Afghan leaders and U.S. military commanders for a first-hand assessment, reports CBS News chief foreign affairs correspondent Lara Logan.
Jalalabad lies near the Tora Bora mountains where al Qaeda leaders fled to and faced a U.S. bombardment during the U.S.-led invasion that ousted the Taliban regime in late 2001 after the Sept. 11 attacks.
Obama and others in the delegation received a briefing inside the U.S. base from the Afghan provincial governor of Nangarhar, Gul Agha Sherzai, a no-nonsense, bullish former warlord.
"Obama promised us that if he becomes a president in the future, he will support and help Afghanistan not only in its security sector but also in reconstruction, development and economic sector," Sherzai told The Associated Press.
The Illinois senator arrived Saturday as part of an official congressional delegation, but the Afghan visit, Obama's first and coming less than four months before the general election, was rich with political implications. Republican presidential rival John McCain has criticized Obama for his lack of time in the region. Obama is also expected to stop later in Iraq.
Logan, traveling with the delegation in Kabul, said that by visiting Afghanistan before Iraq, Obama is signaling the primary importance of the war in Afghanistan in his campaign.
Obama has criticized the Bush administration for making Iraq the focus of American foreign policy rather than Afghanistan, which he deems a grave mistake.
Obama wants to make Afghanistan the focus of the U.S. war on terror and has promised more aid, money and more troops, reports Logan. It's the centerpiece of his foreign policy strategy for the presidential campaign.
Robert Gibbs, a campaign spokesman, said Obama arrived in Kabul around noon. En route from Washington, he made a stopover in Kuwait to meet with U.S. forces stationed there, Gibbs said.
Lt. Col. Bill Nutter, a spokesman for the U.S. military in Kuwait, said Obama "talked to soldiers and constituents and met with senior military leadership."
During the two-hour visit, the officers gave him an overview of operations, Nutter said. Obama shook hands, answered questions, posed for photos and played a little basketball during the visit.
Sultan Ahmad Baheen, spokesman for Afghanistan's Foreign Ministry, confirmed the senator was in Afghanistan and that he would meet with President Hamid Karzai.
"I look forward to seeing what the situation on the ground is," Obama told a pair of reporters who accompanied him to his departure from Andrews Air Force Base on Thursday. "I want to, obviously, talk to the commanders and get a sense both in Afghanistan and in Baghdad of, you know, what the most, their biggest concerns are, and I want to thank our troops for the heroic work that they've been doing."
Obama had recently chided Karzai and his government, saying it had "not gotten out of the bunker" and helped to organize the country or its political and security institutions.
(Left: Sen. Barack Obama is photographed with Gul Agha Shirzai, the governor of Afghanistan's Nangarhar province, center rear, and U.S. military personel in Jalalabad, July 19, 2008.)
Underscoring the challenges in Afghanistan, authorities reported Saturday that a roadside bomb killed four policemen in the volatile south of the country where the Taliban-led insurgency is intensifying nearly seven years after a U.S.-led invasion ousted the militant movement from power.
The Taliban and al Qaeda-linked militants also have caused problems in Afghanistan's east, especially near the border with Pakistan. Nangarhar's Sherzai, considered a no-nonsense, bullish former warlord-turned-governor, briefed the delegation Saturday about the situation in his region, his chief of staff said.
"Barack Obama expressed support for Afghanistan and especially for Nangarhar province," Massoud Ahmad Azizi said. "He said he will support reconstruction, development and security all over the country, especially in Nangarhar. (Obama) thanked Sherzai for good leadership and good administration of the province."
Obama advocates ending the U.S. combat role in Iraq by withdrawing troops at the rate of one to two combat brigades a month. But he supports increasing the military commitment to Afghanistan, where the Taliban has been resurgent and Osama bin Laden is believed to be hiding.
"Because Barack Obama has proposed an increase in U.S. troops to Afghanistan - and because the conflict on the Afghanistan-Pakistan border has intensified - the senator's foreign trip begins with a focus on his aggressive approach to the war on terror, not on Iraq," said CBS News Foreign Affairs Analyst Pamela Falk.
"The Middle East leg of the trip will be more delicate and the European tour more popular," Falk said. "By arriving in Afghanistan first, Obama is making a statement about his priorities."
Also on his travel itinerary is a meeting with Nouri al-Maliki, the Iraqi leader. In an interview published Saturday in the German magazine Der Speigel,.
On the campaign trail, Obama has said one benefit of withdrawing U.S. troops is that it would pressure al-Maliki to shore up his government as well.
Nonetheless, he said he did not plan to reiterate those messages in person.
"I'm more interested in listening than doing a lot of talking, and I think it's very important to recognize that I'm going over there as a U.S. senator," he said. "We have one president at a time."
"This trip gives Obama an opportunity to gather some first-hand intelligence to support the proposals he has already laid out," said CBSNews.com senior political editor Vaughn Ververs. "More importantly, the images and coverage of the trip can give him the imprimatur of leadership critics say is lacking.
"It also carries risks in that any missteps or mistakes will be magnified by the intense coverage," Ververs said.
In a speech this week, Obama said the war in Iraq was a distraction, unlike the fighting in Afghanistan.
"This is a war that we have to win," he said. "I will send at least two additional combat brigades to Afghanistan, and use this commitment to seek greater contributions - with fewer restrictions from NATO allies.
"I will focus on training Afghan security forces and supporting an Afghan judiciary, with more resources and incentives for American officers who perform these missions."
By contrast, his opposition to the war in Iraq - and call for an end to the U.S. combat role - helped him overcome his rivals in the battle for the Democratic presidential nomination.
Lately, his efforts to explain how he will use what he learns from U.S. commanders to refine his proposals have brought charges from Republicans and complaints from Democratic liberals that he seems to be shifting his Iraq policy toward the political center. But Obama maintains his basic goal of ending the U.S. combat role soon remains unchanged and that he's always said the U.S. withdrawal must be done carefully.
Obama also arranged to visit Jordan, Israel, Germany, France and England, traveling aboard a jet chartered by his presidential campaign, before his return to the United States. The weeklong trip marks his only foreign excursion as a presidential candidate; McCain has visited Canada, Colombia and Mexico, in part to highlight Obama's opposition to trade deals with those allies.
Few citizens in impoverished Afghanistan were aware of Obama's unannounced visit, and few have been following the U.S. presidential race, being too busy eking out an existence amid soaring violence and with limited access to news media.
But some interviewed Saturday said they would welcome an Obama presidency if he could help their country end the fighting, corruption and poverty that have crippled it for so long.
"Obama is a good person," said Abdul Basir, 40, a former army officer. "During his campaign I heard he was saying that if I become president I will withdraw the U.S. troops from Iraq and bring them to Afghanistan and I will attack on the terror center on other side of border (in Pakistan). It is very important and I appreciated that."
Obama began his trip with as much secrecy as a presumed presidential nominee can muster.
The senator took an unmarked, corporate Gulfstream-III jet, much smaller than his normal campaign plane, from Chicago to Washington. He was joined by his Secret Service detail, spokeswoman Linda Douglass and two reporters.
Obama deplaned at Reagan National Airport in Washington, took one question apiece from the reporters, and then his motorcade departed for a hasty ride to Andrews Air Force Base about 10 miles away in Maryland.
Upon his arrival, Obama was greeted by a group of Air Force personnel at the bottom of stairs leading to the military Boeing 737 transporting his congressional delegation. Obama's traveling companions, Sens. Chuck Hagel, R-Neb., and Jack Reed, R.I., were not visible to the reporters, but Douglass confirmed they were already on board the aircraft.
Both senators, each a veteran, have been mentioned as potential Obama vice presidential running mates, but Reed has said he's not interested in the job.
As Obama boarded the plane, luggage in hand, a pair of uniformed Air Force officers at the foot of the stairs saluted simultaneously, as they do each time President Bush boards Air Force One.
The only staff member to accompany Obama was a Senate foreign policy aide, Mark Lippert. He is a Navy reservist who returned in late spring from a tour of duty in Iraq.