Britain, Germany and Turkey have offered their troops
Speaking during a visit to Central Asia, the general said Britain was one of several nations offering to lead the international force he hoped would enter the country after the interim government takes office on 22 December.
The size, make-up and leadership of the force have not yet been decided.
But the UN Security Council has agreed it wants the troops to secure the capital Kabul and other main cities before helping relief efforts by aid organisations.
Britain, Germany and Turkey have all been mentioned as candidates for the leadership position.
Awaiting orders
Some British special forces are already in Afghanistan and have been involved in key battles against the crumbling regime.
But there have been reports from French news services that the Northern Alliance are rejecting any British troop deployments even in a peacekeeping capacity.
The Ministry of Defence (MoD) confirmed that Britain was ready to make "a significant contribution" to any international peacekeeping force, but said no commitment had yet been made.
A spokesman said: "We have said from the outset that we could contribute to a security force, but no decisions have been made at this point."
He dismissed press reports that troops from 2nd Battalion Parachute Regiment could be patrolling the streets of Afghan capital Kabul within days.
About 400 British troops from the 2 Para and the 16 Air Assault Brigade are on "heightened readiness for action" to go to Afghanistan.
Hunt continues
Thousands of other troops and supporting personnel are ready to deployed at a week's notice.
About 100 Special Boat Service troops were sent to Bagram airport after the fall of Kabul.
British special forces also helped end an uprising by Taleban prisoners held in a fort in the northern city of Mazar-e-Sharif.
The United States, meanwhile, has indicated that it does not want to lead a peacekeeping force.
General Powell said: "A lot will depend on how many troops they ultimately decide are needed, and then what the sources are for those troops.
"I don't think I want to start speculating on numbers, or numbers of countries, or ethnic, racial or religious breakdowns."
The general's comments came as the hunt continued for Osama Bin Laden and the Taleban's supreme leader Mullah Mohammed Omar.
The man chosen to lead Afghanistan's interim government, Hamid Karzai, promised to bring the two men to "international justice".