Four Americans left Afghanistan by land on Monday with Taliban knowledge in the first departures arranged by the US government since its military pullout, a US official said.

The four US citizens left by land and were greeted by American diplomats, said a senior official, without specifying to which country they crossed.

"The Taliban did not impede them" and were aware of the effort, the official told reporters accompanying Secretary of State Antony Blinken flying to Qatar.

US officials say other Americans may have left since the United States ended its 20-year war at the end of August, but they would have done so by private means.

Washington says it is closely watching whether the Taliban makes good on promises to let US citizens and allies depart as it decides how to deal with the Islamists, who seized the capital Kabul on August 15 as the US-backed government crumbled.

US officials say just over 100 Americans, mostly dual nationals, remain in Afghanistan after the massive airlift of tens of thousands of people in the last days of America's longest war.

President Joe Biden's Republican rivals have been quick to accuse him of abandoning Americans. 

Tens of thousands of interpreters or others who supported the US mission and their family members are believed to remain, with many fearing retribution despite Taliban assurances. 

With Kabul airport in disarray, land routes are the key way out of Afghanistan, primarily though Pakistan or Iran, which does not have diplomatic relations with Washington.