All U.S. and NATO troops have left the biggest air base in Afghanistan, officials said Friday, signalling the complete withdrawal of foreign forces from the country was imminent after two decades of war. 

"The American and coalition forces have completely withdrawn from the base and henceforth the Afghan army forces will protect it and use it to combat terrorism," defence ministry spokesman Fawad Aman said in a tweet.

A U.S. defense official, speaking to Reuters on condition of anonymity, also confirmed the withdrawal of the American troops from their main military base.

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The U.S. military has coordinated its air war and logistical support for its Afghan mission from the Bagram air base, about 60 km north of Kabul, and the withdrawal of the forces symbolizes the end of the U.S. military involvement in the country.

The base is being handed over to the Afghan government as its armed forces face a surging war with the Taliban and questions swirl about their prospects.

An Afghan official said the base would be officially handed over to the government at a ceremony on Saturday.

Last month, U.S. President Joe Biden told his Afghan counterpart, Ashraf Ghani, that "Afghans are going to have to decide their future, what they want."

Ghani said his job was now to "manage the consequences" of the U.S. withdrawal.

The agreement with the Taliban on the U.S. pull-out was struck under the administration of President Donald Trump. 

In exchange for the U.S. withdrawal, the Taliban, fighting to expel foreign forces and oust the U.S.-backed government, have vowed to prevent any international terrorism from taking root on Afghan soil.

They also made a commitment to enter into talks with their Afghan rivals but little progress has been made in negotiations.

(With input from agencies)