U.S. troops are likely to remain in Afghanistan for many months, and not just to finish the job of destroying Usama bin Laden's terrorist forces and his Taliban collaborators.

Defense Secretary Donald H. Rumsfeld says a U.S. military presence is needed also to help a new Afghan government get on its feet and make the difficult transition to a permanent, viable peace.

``We want to do what is appropriate for us to do to help them get through what is clearly a difficult period,'' Rumsfeld said Thursday. He offered no specific estimate of how long that period would last, but he made it clear that U.S. forces would remain on the ground at least until summer.

``How long it will be, I just don't know,'' he told a Pentagon news conference. ``You can be sure we're not going to stay there a second longer than we have to, but we also feel an obligation to be a responsible nation and a participant in this process and help them navigate through what has to be an enormously difficult thing to do.''

He noted that the interim government that took office Dec. 22 is due to hand off to a more permanent power structure in June. It faces many complex issues, Rumsfeld said, including how to organize and unify an Afghan military in a country that has been plagued by tribal conflicts for centuries.

``Expecting that to happen smoothly I think is unrealistic,'' he said.

Rumsfeld's description of an American military role in propping up the Afghan government suggests that he is now looking beyond the combat and humanitarian missions that began more than three months ago.

Rumsfeld resists the idea of using U.S. forces for what some call ``nation building'' — the kind of house-raising, crime-chasing, checkpoint-monitoring missions that the Clinton administration committed the U.S. military to in Bosnia and Kosovo. Rumsfeld has criticized that open-ended commitment of military forces.

Yet his comment about ``contributing to the security environment'' in Afghanistan by having a visible, lasting military presence in key cities suggests Rumsfeld is thinking that a longer stay may be necessary.

Rumsfeld noted that a British-led international security force is operating only in Kabul, the capital. U.S. troops are not part of that force.

In his public comments about the future of the U.S.-led military campaign, Rumsfeld has begun referring more explicitly to his concerns about the political stability of an Afghanistan wracked by decades of war.

He referred Thursday to the uncertainty of Afghanistan's political lineup after the interim government is replaced. The head of the interim government, Hamid Karzai, is a pro-American leader who worked closely with U.S. troops to defeat the Taliban. He still talks to Rumsfeld regularly.

``The fact remains that this government ends at a certain point in the immediate future ... and the new government comes in,'' Rumsfeld said. ``And will they do it the same way the interim government did?''

The United States has about 4,000 troops in Afghanistan and many hundreds more in neighboring Pakistan and Uzbekistan. Smaller numbers are in Tajikistan and Kyrgyzstan, and thousands are aboard Navy ships patrolling the Arabian Sea to block any sea route of escape for bin Laden.

U.S. warplanes fly missions daily over Afghanistan from land bases in the area and from aircraft carriers in the Arabian Sea.

The main American ground force in Afghanistan is the Army's 101st Airborne Division, which has taken over for the Marines at Kandahar airport, where more than 300 Al Qaeda and Taliban prisoners are held.

For now, the U.S. troops are busy trying to eliminate remaining pockets of al Qaeda and Taliban resistance and hunting bin Laden and the Taliban's deposed leader-in-hiding, Mullah Mohammed Omar. Rumsfeld said the mission will continue to require the firepower of American ground troops.

``We keep fighting them, and we intend to keep doing that,'' he said. ``That takes presence. You can't do that from Chicago. You have to be in there and we have to be present.''

The goal is to capture Omar and bin Laden ``if we can,'' Rumsfeld said, ``to keep them from conducting additional terrorist acts on the one hand and to keep them from turning Afghanistan back into a haven for terrorists.''