The United States started evacuating its diplomats on Sunday and is sending additional troops to help secure Kabul airport and the embassy after the Taliban overran another two key cities bringing them closer to the Afghanistan capital. 

U.S. President Joe Biden on Saturday said, "Based on the recommendations of our diplomatic, military and intelligence teams, I have authorized the deployment of approximately 5,000 U.S. troops to make sure we can have an orderly and safe drawdown of U.S. personnel and other allied personnel."

This is an additional 1,000 men as earlier the number of troops approved for the evacuation was put at 4,000. The US is also flying a backup force to nearby Kuwait in case the situation gets out of hand and additional troops are required for an orderly draw down from Kabul.

Biden said his administration had told Taliban officials in Qatar that any action that put U.S. personnel at risk "will be met with a swift and strong U.S. military response."

But he also said an indefinite U.S. military presence was not an option.

President Biden has defended his decision to escalate the US withdrawal from Afghanistan. "One more year, or five more years, of U.S. military presence would not have made a difference if the Afghan military cannot or will not hold its own country. And an endless American presence in the middle of another country's civil conflict was not acceptable to me," Biden said.

Similarly, other NATO countries such as the UK and Germany have brought in their own troops to help in the evacuation.

Also read:  Taliban capture all major cities in Afghanistan, only Kabul remains

Terrified refugees were reported to be flowing into Kabul to escape the wrath of the advancing insurgents. 

Hundreds of people slept huddled in tents or in the open in the city, by roadsides or in car parks, Reuters quoted a Kabul resident as saying.

Western governments were accelerating plans to evacuate their embassy staff, citizens and Afghans who had worked for them. The British ambassador will leave the country by Sunday evening, UK media reported.

An Iranian official said the embassy in Kabul would be evacuated by Monday.