Rebuilding Afghanistan into a stable nation won't be easy, Defense Secretary Donald H. Rumsfeld says, but the United States wants ``to be as helpful as we can.''

Rumsfeld, making the first trip by a top U.S. official to this country newly freed from the rule of the Taliban militia, met with the interim prime minister, Hamid Karzai, on Sunday at an airfield bearing the scars of decades of war. 

Rumsfeld also told U.S. troops that their task is to ensure terrorists face punishment for the Sept. 11 attacks. The defense chief addressed U.S. military forces here and at another base near Afghanistan whose location could not be revealed under ground rules established by the Pentagon for reporters traveling with Rumsfeld. 

There's no way to know how long it will take to finish the job and find Osama bin Laden, he said. 

Rumsfeld was to confer Monday with Russian Defense Minister Sergei Ivanov in Brussels, Belgium, before a NATO defense ministers' meeting. 

Russia has said it was disappointed by President Bush's announcement last week that the United States will pull out of the 1972 Anti-Ballistic Missile treaty. But President Vladimir Putin also said the move does not threaten his country. 

The United States announced its decision to pull out of the treaty because it wants to develop a national missile defense system. 

Bush tried to strike a deal with Putin to allow the United States to expand testing for such a system. But Russia, which can't afford a national missile defense, has said it views the ABM pact as the basis of all nuclear-reduction treaties. 

Rumsfeld has robustly defended the decision. He told the leaders of former Soviet republics he visited over the weekend that the U.S. decision would not create an arms race but help stabilize the world. 

Secretary of State Colin Powell also said Sunday that good relations with Russia will continue despite the ABM pullout decision. 

Russia and the United States have announced plans to cut their nuclear warhead stockpiles significantly. The United States says it will reduce its number of warheads to 2,000 or less from the current 6,000. Russia has made a similar pledge. 

Russia has worked closely with the United States on fighting terrorism since the Sept. 11 attacks. Putin has made clear he expects that close relationship to continue. 

In Afghanistan, Rumsfeld told reporters that Karzai, the incoming prime minister, ``is anxious to be cooperative with us in every possible way.'' 

Nevertheless, ``It's not going to be an easy task,'' to build Afghanistan into a country without factional fighting and inhospitable to terrorists, the defense secretary said. ``We want to be as helpful as we can'' in that regard, Rumsfeld said he told Karzai. 

The United States could not, and does not want to, try to itself create a stable government. Instead, it wants to help Karzai and others, Rumsfeld said. 

Sitting less than a mile from former Taliban front lines, on folding chairs in a room draped with camouflage inside a damaged aircraft hangar, Karzai told Rumsfeld the U.S. military had boosted an opposition ``incapacitated'' by years of war. 

``The way you provided help for us was the opportunity that we wanted,'' Karzai said. 

Rumsfeld also met with the interim government's new defense minister, Mohammed Fahim. His actions once U.S. troops leave will be ``enormously important'' in determining the country's future, the secretary said. 

An international security force of between 3,000 and 5,000 troops from various countries will enter the Afghan capital of Kabul sometime after Dec. 22, the day Karzai takes office, Rumsfeld said. 

The United States will provide support, including intelligence, airlift support and a rapid reaction force in case of trouble. There are ``rumblings'' that a similar security force for one or two other Afghan cities is under consideration, he said.