US President Joe Biden is sticking to the US timetable of an Aug 31 withdrawal from Afghanistan, he told G7 leaders on Tuesday, while the Pentagon announced that 4,000 Americans have been evacuated so far from the war-riddled country.

The decision goes against the preference of some allied leaders, who wanted more time for the evacuations.

"We will go on right up until the last moment that we can," said British Prime Minister Boris Johnson, who had pushed to keep the Kabul airport presence after Aug 31. Johnson acknowledged he was unable to persuade Biden to extend the US presence.

"But you've heard what the president of the United States has had to say, you've heard what the Taliban have said," Johnson said.

Professor Malcolm Chalmers, deputy director-general of the Royal United Services Institute (RUSI) think tank, told Newsweek that as part of the "special relationship" with the Americans, British leaders expected to be consulted on the Afghanistan withdrawal. 

"The withdrawal has punctured common illusions on the extent of Britain's military dependence on the United States," Chalmers said. "British leaders had convinced themselves that they had real influence in successive wars in the Balkans, Iraq and Afghanistan. Now the reality of dependence without influence has been exposed."

Chalmers said that one result of the US move "could be that the UK government will take European defense cooperation more seriously than it has for many years. This could be one of the few positive side-effects from this sorry debacle," he said.

Biden said the Taliban, who took control of the country at lightning speed over several days, are cooperating despite some violent incidents.

"But it's a tenuous situation," he said. "We run a serious risk of it breaking down as time goes on."

The Pentagon said 21,600 people had been evacuated in the 24 hours that ended Tuesday morning, and Biden said an additional 12,000 had been flown out in the 12 hours that followed. Those include flights operated by the US military as well as other charter flights. Biden said US forces have helped evacuate 70,700 people since Aug 14.

Later Tuesday, the president said he had asked the Pentagon and State Department for evacuation contingency plans that would adjust the timeline if necessary.

The Taliban, who have retaken control of the mountainous Asian country nearly 20 years after being ousted in a US-led invasion after the Sept 11, 2001, terrorist attacks, insisted the airlift must stop on Aug 31.

Taliban spokesman Zabihullah Mujahid said "after that (Aug 31), we won't let Afghans be taken out" on evacuation flights. He also said the Taliban would prevent Afghans from accessing roads to the airport.

Mujahid said the main problem was the chaos at the airport, and he accused the US of luring Afghan engineers, doctors and other professionals.

Afghans flocked to the Kabul airfield last week, and some latched on to a US military transport plane, later plunging to their deaths. At least seven people died that day, and another seven died Sunday in a stampede. An Afghan solider was killed Monday in a gunfight.

In Washington, Pentagon spokesman John Kirby said Aug 31 is enough time to evacuate all Americans but was less specific about completing the evacuation of Afghans. 

"As of today, August 24, we have evacuated approximately 4,000 American passport holders plus their families. We expect that number to continue to grow in the coming days," Kirby said.

Former US president Donald Trump said in a statement Tuesday, "Now we are learning that out of the 26,000 people who have been evacuated, only 4,000 are Americans."

Biden said his administration was working to rebuild a system for processing refugees that he said was "purposely destroyed" by Trump.

"We must all work together to resettle thousands of Afghans who ultimately qualify for refugee status,"  he said.

United Nations human rights chief Michelle Bachelet told an emergency session of the UN Human Rights Council in Geneva that "a fundamental red line will be the Taliban's treatment of women and girls".

David Beasley,  executive director of the World Food Programme, said the political situation must be resolved soon because the conflict, drought and the pandemic mean that 14 million Afghans could face starvation.

The Associated Press and Reuters contributed to this story.