On Saturday, the last UK plane carrying solely civilians left Kabul, leaving hundreds of Afghans eligible for resettlement behind.

The government is scaling up its airlift of civilians, diplomats, and military out of Afghanistan before the US troop withdrawal deadline on August 31.

The UK government said that the additional planes might transport civilians as well as military and diplomatic personnel.

The evacuation effort would end "during the course of the day," according to General Sir Nick Carter, the head of the UK armed forces, who told the BBC on Saturday.

"We have some civilian flights to take out, but it's very few now," Carter said, after which "it will be necessary to bring our troops out on the remaining aircraft."

The UK Ambassador to Afghanistan, Laurie Bristow, tweeted a video of himself standing on the airfield with military planes in the background, wearing a bullet-proof vest.

He said his team had been working "to the very last moment to evacuate British nationals, Afghans, and others at risk." 

"Since August 13, we have brought nearly 15,000 people to safety," the ambassador said.

Carter said the evacuation operation had "gone as well as it could do," but it was "heartbreaking" that "we haven't been able to bring everybody out."

The armed forces chief estimated the number of eligible Afghans who had not been evacuated to be "in the high hundreds." He stressed that they will be welcome in Britain if they manage to leave after the deadline, through third countries or other ways.