A potential showdown looms between the United States and the Taliban over the scheduled withdrawal of American forces from Afghanistan by Aug 31. 

Taliban spokesman Suhail Shaheen, in an interview Monday with Sky News, said an extension beyond that date would not be acceptable.

When asked if the Taliban would agree to the US or UK being given more time for evacuations, Shaheen said "no".

"You can say it's a red line. President Biden announced this agreement, that on the 31st of August they would withdraw all their military forces. So if they extend it, that means they are extending occupation, while there is no need for that. … It will deteriorate the relation ... that will create mistrust between us. If they are intent on continuing the occupation, so it will provoke a reaction," he said.

The date presents a dilemma for US President Joe Biden, who has pledged to evacuate every American who wants to leave Afghanistan.

Aug 31 also is less than two weeks from the 20th anniversary of the Sept 11 terrorist attacks in the US, the initial reason for the US military going to Afghanistan.

Reuters reported Monday that Biden is expected to decide as soon as Tuesday on whether to extend the deadline. The news outlet cited an official in the administration, however, as saying some of the president's advisers were arguing against extending the deadline for security reasons.

Biden will hold talks with leaders from the Group of Seven on Tuesday. British officials have said that Prime Minister Boris Johnson will use the emergency meeting to propose new sanctions on the Taliban and persuade the US president to extend the evacuation operation, The Washington Post reported. 

Ben Wallace, Johnson's defense minister, has said that Great Britain, like other nations dependent on US air and ground support in Afghanistan, will have to halt its own evacuation efforts when US forces leave.

"It's really important for people to understand the United States have over 6,000 people in Kabul airport, and when they withdraw, that will take away the framework … and we will have to go as well," Wallace said, according to the Post.

There is continuing chaos at the airport in Kabul, from where the evacuation flights arrive and leave. Tens of thousands of people have flocked to the airfield, which has sustained sporadic violence.

A firefight just outside the airport killed at least one Afghan soldier early Monday, German officials said. The activity around the airport includes gunfire, beatings by the Taliban, while some caught up in the desperate crowds have been trampled.

Taliban spokesman Zabihullah Mujahid, in addressing a conference of Muslim clerics, urged them to push back against Western "propaganda" and said the US was undermining Taliban rule by sending planes and offering Afghans asylum.

Mohammad Khalid, another Taliban official who addressed the same gathering, said "history and Afghans will not forgive those who were trained in the US and Europe and returned to kill their own people".

Twenty-eight US military flights transported about 10,400 people out of Afghanistan over the 24 hours that ended early Monday morning, a White House official said.

The US says it has evacuated and facilitated the evacuation of about 37,000 people since Aug 14. The military says it has the capacity to fly between 5,000 and 9,000 people out per day.

The Biden administration has given no clear estimate of the number of Americans seeking to leave Afghanistan. Some have put the total between 10,000 and 15,000.

At a press briefing Monday, White House press secretary Jen Psaki objected to a reporter's characterization that Americans were "stranded" in Afghanistan.

"First of all, I think it's irresponsible to say Americans are stranded. They are not," Psaki replied. "We are committed to bringing Americans who want to come home, home. We are in touch with them via phone, via text, via email, via any way that we can possibly reach Americans to get them home if they want to return home."

John Kirby, the chief Pentagon spokesman, said the faster pace of evacuation was partly due to coordination with Taliban commanders on getting evacuees to the airport.

"Thus far, and going forward, it does require constant coordination and deconfliction with the Taliban," Kirby said. "What we've seen is, this deconfliction has worked well in terms of allowing access and flow as well as reducing the overall size of the crowds just outside the airport."

"We are in talks with the Taliban on a daily basis through both political and security channels," White House national security adviser Jake Sullivan said, adding that it will ultimately be Biden's decision alone whether to continue military-led evacuation operations past Aug 31.

The Pentagon said it has added a fourth US military base, in New Jersey, to three others (in Virginia, Texas and Wisconsin) to temporarily house arriving Afghans.

Major General Hank Williams, the joint staff deputy director for regional operations, told reporters there are now about 1,200 Afghans at those military bases. The four bases combined can hold up to 25,000 evacuees, Kirby said.

Elsewhere in Afghanistan, the Taliban have faced limited armed resistance from fighters in Baghlan province, some 75 miles (120 kilometers) north of the capital Kabul. The anti-Taliban fighters claimed to have seized three districts in the Andarab Valley on Sunday, but the Taliban said Monday that they had cleared them out overnight.

Mujahid, the Taliban spokesman, said the group's forces have also surrounded nearby Panjshir, the only one of Afghanistan's 34 provinces yet to fall to the Taliban. Several Taliban opponents have gathered there, vowing to resist any attempt to take the province by force.

Mujahid said there had been no fighting in Panjshir and that the Taliban are seeking a "peaceful solution".

The Associated Press contributed to this story.