Mullen: Taliban-Pakistan Link "Growing"

Updated at 7:43 a.m. Eastern.

America's top military officer is expressing concern about the "growing level of collusion" between Taliban insurgents in Afghanistan and al Qaeda and other militants across the border in Pakistan.

Adm. Mike Mullen told reporters Monday that he will discuss the issue with Pakistani authorities during talks in Islamabad later this week. Mullen made the comments after arriving in Kabul to discuss the upcoming U.S. troop surge.

CBSNews.com Special Report: Afghanistan

Painting a grim picture, the chairman of the Joint Chiefs of Staff says Afghan insurgents are dominant in a third of Afghanistan's provinces and he is "deeply concerned about the growing level of collusion" between Afghan militants and extremists "taking refuge across the border in Pakistan."

The Pentagon's top military officer was visiting Afghanistan just as the first of the 30,000 U.S. reinforcements ordered by President Obama are starting to deploy to the 8-year-old war.

The Joint Chiefs of Staff chairman said last week that 16,000 troops have received their orders for Afghanistan since President Obama announced his new war strategy. The first to be deployed - a battalion of Marines - are to arrive in southern Afghanistan this week. Tens of thousands of tons of construction materials, winter gear and other equipment also are in the pipeline.

President Obama told "60 Minutes" correspondent Steve Kroft in an interview broadcast Sunday evening that the decision to send 30,000 more young Americans into harm's way in Afghanistan was "absolutely" the most difficult decision of his presidency to date.

President Obama on "60 Minutes":

Obama Defends Afghanistan Timetable
Transcript: President Obama, Part 1
Transcript: President Obama, Part 2
Obama Versus the "Fat Cats"
Obama: Gatecrashers Lapse "Won't Happen Again"
Obama: Senate Will Pass Health Bill by Christmas

He acknowledged the waning support for the ongoing war effort in the American public, and even among his fellow Democrats, but told Kroft, he believed it was, "the right thing to do. And that's my job. If I was worried about what polled well, there are a whole bunch of things we wouldn't have done this year."

Mullen is one of a host of top military officials and world leaders to visit the country following the announcement of the new U.S. strategy for Afghanistan, which includes a plan to begin pulling troops out in July 2011. All the visitors have sought to reassure Afghan officials that international forces would not abandon the nation in 18 months.

In a visit to the war zone last week, U.S. Defense Secretary Robert Gates told Afghanistan's senior military officials that while the U.S. looks forward to the day when the Afghans can take control of their country, the United States would have a large number of forces in Afghanistan for some time beyond July 2011. "This is a relationship forged in blood," he said. "We will see it to the end."

Col. Wayne Shanks, a spokesman for the international force, said U.S. troops will begin arriving over the next week or so.

"By the middle of summer, you should see most of the forces that have been pledged arrive here in Afghanistan," Shanks told a joint NATO-Afghan press conference shortly after Mullen arrived.

Shanks said the fresh troops would be sent mainly to the south, but would not disclose exact locations.

Obama ordered the troop surge to try to curb the burgeoning Taliban insurgency as the bloodiest year of the Afghan war draws to a close.

Underscoring the security crisis, Afghanistan's Ministry of Interior announced that 16 Afghan National Police were killed Monday in two separate attacks - one in northern Baghlan province and the other in the southern city of Lashkar Gah.