The order came on Tuesday as the US special envoy for Afghanistan told lawmakers that it no longer made sense to continue the 20-year deployment of American troops there.

At the same time, Ambassador Zalmay Khalilzad said he shared lawmakers' concerns that the rights of women and minorities could be jeopardised after the withdrawal is complete.

"We should all remain concerned that those rights could suffer," Khalilzad told the Senate Foreign Relations Committee.

Asked if the US would retain any leverage to protect those rights once its troops are gone, Khalilzad was circumspect. He said aid and other types of diplomatic support "would be not available if they did not respect the human rights of Afghan women or others".

Shortly before he spoke, the State Department said it had instructed all personnel to depart unless their jobs require them to be physically located in Afghanistan.

The order was not specific as to the number of people affected, but it went well beyond the usual curtailment of staffers for security and safety reasons. Such orders normally apply only to non-essential personnel.

In an updated travel advisory for Afghanistan, the department said it had ordered the departure of all US government employees "whose functions can be performed elsewhere".

It also said American citizens should not travel to Afghanistan and those there who want to depart "should leave as soon as possible on available commercial flights".

The embassy in Kabul is heavily dependent on the US military for security, and staff drawdowns had been been underway since the Trump administration had announced last year that American troops would be withdrawn from Afghanistan by May 1.

The Biden administration extended that deadline until September 11, the 20th anniversary of the 2001 terrorist attacks, but has accelerated the pullout.

The State Department order came just two days after General Austin Miller, America's top general in Afghanistan, said the US military had begun closing down operations in the country and that Afghanistan's security forces had to be ready to take over.

While the official start to the withdrawal of Washington's 2500 to 3500 troops and NATO's 7000 allied forces is May 1, Miller said the pullout had already begun.

In February last year, the US military began closing its smaller bases. In mid-April, the Biden administration announced that the final phase of the withdrawal would begin May 1 and be completed before September 11.

The pullout of American troops and their NATO coalition colleagues is proceeding even in the absence of a peace deal between the Afghan government and the Taliban, whose harbouring of Osama bin Laden'a al-Qaeda network prompted the US invasion of Afghanistan after 9/11.

Negotiations between the Taliban and the government have been at a standstill for some time and talks won't resume until next month, raising fears that the withdrawal of foreign forces may lead to a resumption in all-out civil war.

Australian Associated Press