U.S. military commanders have drafted plans to lower the number of American troops in Afghanistan by roughly 20 percent next year if NATO-led troops from Europe continue to widen their role in securing the country, according to senior officers here.

A reduction of as many as 4,000 of the nearly 20,000 American troops in Afghanistan would be the largest drop in a force that generally has grown since the U.S.-led invasion in late 2001 drove the Taliban from power and routed al Qaeda fighters.

The U.S. plan is contingent on an improving political and security situation in Afghanistan and on decisions still to be made by NATO about what types of forces it would commit and its willingness for them to engage in combat against insurgents, the officers said.

"It makes sense that as NATO forces go in, and they're more in number, that we could drop some of the U.S. requirement somewhat," Army Gen. John P. Abizaid, the senior U.S. commander in the region, said in an interview here. He stressed that no decision had been made to shrink the U.S. military presence.

The planning comes as intensified fighting in Afghanistan this year has killed more than 50 Americans, the highest death toll in any year since the troops arrived. U.S. officials say the increase is due in large part to aggressive moves by U.S. forces to root out guerrillas.

Although U.S. commanders expect the fighting to subside when harsh winter weather sets in, they anticipate another round of battles starting in the spring. "I think you have to get used to the cycle of fighting in Afghanistan," Abizaid said.

Even so, he and other commanders expressed confidence that the threat posed by the Taliban movement will dwindle as the Afghan government's authority spreads and Afghan forces become more capable.

NATO already leads a peacekeeping contingent of more than 9,000 troops known as the International Security Assistance Force, whose mandate was renewed for another year on Tuesday by the U.N. Security Council. In the past two years, the force has broadened its reach from its original base in Kabul, the capital, and now commands operations in the country's northern and western regions.

The alliance is moving toward assuming command in the southern region by May, which would leave the U.S. military responsible only for the eastern region.

But the expansion remains a sensitive issue in the alliance, particularly among member countries that have been reluctant to put their troops into combat as opposed to the less risky job of peacekeeping in relatively secure areas.

U.S. troops have so far borne the brunt of the fight with Taliban insurgents, whose strongholds are in remote parts of the south and the rugged hills of eastern provinces bordering Pakistan.

Defense Secretary Donald H. Rumsfeld, arriving in Berlin on Tuesday for a meeting of NATO defense ministers, urged the alliance to shoulder more of the work in Afghanistan and to better develop its counterinsurgency and counter-narcotics capabilities. The NATO ministers plan to discuss Afghanistan on Wednesday.

The NATO force being readied for the south will consist of troops from Britain as well as the Netherlands and Canada, countries with records of combat operations since 2001.

British Defense Secretary John Reid appeared to back Rumsfeld's message after talks with the Pentagon leader, telling reporters Tuesday that the alliance must "have forces which are not paper forces."

However, speaking to reporters in London before heading for Berlin, Reid had said that the expansion of NATO's forces in Afghanistan next year was not intended primarily to deal with insurgents. "Their primary mission will not be to go chasing Taliban all over the country," Reid said, according to news service reports.

Handing more responsibility to NATO would appear to lighten the load on the U.S. military much faster than waiting for Afghanistan's fledgling security forces to become ready to conduct counterinsurgency operations on their own.

The Afghan army has grown to about 25,000 troops, and the Afghan police force has about 50,000 members. But U.S. commanders acknowledged in interviews here in the past two days that the development of both security branches has run into problems.

Concern about continuing shortfalls in logistical and other support for Afghan army units in the field recently prompted Army Lt. Gen. Karl W. Eikenberry, the senior commander in Afghanistan, to slow the formation of new battalions, from about two a month to one.

And a move to set up what officers call "partnering relationships" between Afghan and U.S. units to foster on-the-job training is only now beginning. The first joint operation between an Afghan battalion and an American one occurred last week in the northeastern province of Konar.

"This question of partnering is something we've now aggressively adopted," Eikenberry said, "but perhaps we could have moved on that piece a little bit earlier."

The police suffer serious shortages of weapons, vehicles and communications gear and a reputation for corruption. "It's more or less a hollow force," said Air Force Maj. Gen. John Brennan, who oversees the development effort.

Germany has had the lead in training the police under an international division of responsibility set up after the 2001 invasion. But the Pentagon recently stepped in, broadening the effort by adding a mentoring program using foreign civilian law enforcement veterans and developing Afghan trainers. The United States also has allotted $860 million for police equipment, with the goal of establishing what Brennan called "a credible, well-equipped" force by the end of 2009.

Even if U.S. force levels in Afghanistan drop significantly next year, a sizable U.S. military presence in this country is likely for some years, according to U.S. and military diplomatic officials here.

The United States would continue to have what Eikenberry called "enduring missions" in Afghanistan, including the development of Afghan's security forces and the struggle against the country's vast poppy crop and opium trade.

U.S. forces also will be needed to provide airlift, intelligence and other logistical assistance to NATO allies. "We have to keep a force that's agile enough to back up NATO," said Ronald E. Neumann, the U.S. ambassador here. "NATO is not a question of providing some excuse for us to quit the mission early."

Under a longer-term alliance plan that envisions NATO taking command of Afghanistan's eastern sector, U.S. ground forces would continue to play the lead role there.

With elections due Sunday for a national assembly and provincial councils, the U.S. military command here is bracing for a possible surge in violence, although the level of insurgent activity in recent days has been lower than officers expected.

The potential for danger will not pass with the election and might intensify in the aftermath, U.S. officers here warned. They noted that millions of ballots must be transported from distant villages to central counting centers and that victorious candidates would likely end up waiting several months to be sworn into office.

A woman carries her child through a Kabul graveyard. The U.S. military is bracing for possible violence before Sunday's parliamentary elections.