British troops have left Kabul, ending the UK's evacuation operation and its 20-year military involvement in Afghanistan.

Prime Minister Boris Johnson praised the “heroic” evacuation effort, even as the government acknowledged some eligible Afghan civilians had been left behind. The UK's top military officer conceded that “we haven't been able to bring everybody out."

The UK government said late Saturday that about 1,000 troops who ran an airlift of British nationals and Afghan civilians had departed from Kabul airport, hours after the final evacuation flight for civilians. Most countries apart from the United States had already left.

Before departing, Britain's ambassador to Afghanistan, Laurie Bristow, said from Kabul airport that it was “time to close this phase of the operation now.”

“But we haven't forgotten the people who still need to leave,” Bristow said in a video posted on Twitter. “We'll continue to do everything we can to help them. Nor have we forgotten the brave, decent people of Afghanistan. They deserve to live in peace and security.”

Britain says it has evacuated more than 15,000 people from Kabul in the past two weeks but that as many as 1,100 Afghans who were entitled to come to the UK have been left behind. Some British lawmakers who have been trying to help stranded constituents and their families believe the true total is higher.

“We haven't been able to bring everybody out, and that has been heartbreaking, and there have been some very challenging judgments that have had to be made on the ground,” the head of British armed forces, Gen. Nick Carter, told the BBC.

Foreign citizens from around the world and the Afghans who worked with them have sought to leave the country since the Taliban's swift takeover this month after most US forces departed. About 117,000 people have been evacuated through Kabul airport, according to American officials.