The deaths brought the number of troops who have died in Afghanistan this year to 113, according to an Associated Press tally, surpassing last year's record toll of 111.
Afghanistan was the launching pad for al Qaeda's terrorist attacks on Sept. 11, 2001. U.S. forces invaded in October 2001 in response and quickly drove the Taliban out of power.
Across Afghanistan, U.S. troops paused in silence Thursday to commemorate the 9/11 attacks. At a U.S. base in Kabul, members of the New York National Guard, many of whom served at the site of the World Trade Center after the towers came down, remembered the attack on their home state.
"For those of us who were there, served at Ground Zero, 9/11 is deeply personal," said Col. Brian K. Bale, the commander of the 27th Infantry Brigade Combat Team.
Maj. Stephen Bousquet, 34, of Buffalo, N.Y., provided security at Ground Zero for three weeks after the attack. He now trains and mentors Afghan police, he said, "so American and coalition forces can leave one day."
Osama bin Laden, leader of the al Qaeda network, is believed to be in the lawless tribal belt on the Pakistan-Afghanistan border. He had been sheltered by Taliban leader Mullah Omar before 9/11.
Taliban fighters folded in easy defeat in fall 2001 in what at first appeared to be a resounding U.S. victory. But militants that U.S. commanders once derided as ragtag amateurs have transformed into a fighting force advanced enough to mount massive conventional attacks. Suicide and roadside bombs have turned bigger and deadlier than ever.
The number of Arab, Chechen and Uzbek militants flowing into the Afghan-Pakistan region has increased this year, bringing with them command expertise the Taliban lacked.
U.S. death tolls have climbed sharply from the first years of the war. Only five American service members died in 2001. Thirty service members died in both 2002 and 2003; the toll climbed to 49 in 2004, then 93 in 2005 and 88 in 2006.
Last year 111 troops died, including one killed by a sniper while meeting with Pakistani officers in Pakistan. That mark was surpassed Thursday - with more than three months left in the year - reflecting both the increased number of American troops deployed to Afghanistan as well as the insurgency's increasing potency.
Top U.S. generals, European leaders and analysts say the blame lies to the east, in militant sanctuaries in neighboring Pakistan. As long as those areas remain havens where fighters arm, train, recruit and plot increasingly sophisticated ambushes, the Afghan war will continue to sour.
"What you have is a broad expansion on the front, and because of this you have expansion in areas of the Taliban. Even in areas where there is no substantial fighting, the presence of insurgents has increased," said Anthony Cordesman, a security expert with the Washington-based Center for Strategic and International Studies.
"You have less cooperation from Pakistan, and there's political disarray," which creates a situation where there is little security and stability, Cordesman said. "You also have a weak government that is incapable of maintaining a significant presence in high threat areas."
Since the 2001 invasion, a total of 519 U.S. troops have died in the Afghan war, including those killed in border areas of Pakistan and in Uzbekistan, which was a staging area in 2001. An additional 65 more have died outside the Afghan region in support of Operation Enduring Freedom, according to the Defense Department.
The Pentagon says 117 U.S. service members died last year in Operation Enduring Freedom, but that includes six deaths outside the Afghan region: two in the Philippines, two in Ethiopia, one in Somalia and one in the Gulf.
The NATO-led force in Afghanistan said one soldier was killed Thursday in the east when insurgents attacked a compound. The separate U.S.-led coalition said a second service member died while conducting combat operations. No other details were released, but a Western military official told The Associated Press that both troops were American.
Gen. Mohammad Zahir Azimi, the spokesman for Afghanistan's Ministry of Defense, noted the militants aren't just targeting U.S. forces. He said Afghan soldiers and police have also suffered a record number of casualties over the past year. Figures weren't immediately available.
President Bush announced this week that he was sending an Army brigade and a Marine battalion to Afghanistan in November to replace two that are scheduled to leave.
Some 33,000 U.S. troops are now stationed in the country, the highest level since 2001. Overall, more than 65,000 troops from 40 nations are deployed in Afghanistan.
U.S. commanders in Afghanistan say they need another 10,000 troops - even more than the deployment plan Bush announced. The commanders also urge more nonmilitary aid and say the Afghan government must perform better.
Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, told the House Armed Services Committee this week that,