A defiant Taliban on Monday said that it would not accede to any request by the US-led western powers for an extension of the August 31 deadline for the complete evacuation of Afghanistan.

"If the US or UK were to seek additional time to continue evacuations — the answer is no. Or there would be consequences," Taliban spokesman Suhail Shaheen told Sky News on Monday.

Staying beyond the agreed deadline would be "extending occupation", he added.

While the US had initially sanctioned 3,000 troops to secure the Kabul airport and safely evacuate foreign nationals and at-risk Afghans the number was doubled to 6,000 following the sudden collapse of the Afghan army and the resultant chaos on the ground. US forces have also been put on standby in nearby Kuwait and can be quickly deployed if the need arises. 

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There has been complete chaos outside the airport and people are facing a problem in getting through the Taliban check-points both in Kabul and outside provinces to get to the airport.  At least 20 people have been killed in and around the airport last week.

One Afghan was killed and three others were injured on Monday morning in a fight that erupted between Afghan guards and unknown assailants. German and American troops "participated in further exchange of fire", the German army said in a statement.

Biden has insisted he wants to end the US military presence and the airlifts by August 31. Asked by a reporter whether the United States would extend an Aug. 31 deadline for evacuations, Biden replied: "Our hope is we will not have to extend but there are going to be discussions I suspect on how far along we are in the process."

But with the European Union and Britain saying it would be impossible to evacuate everyone by August 31, the US President is under pressure to extend the deadline.

Germany and Switzerland had to cancel flights out of Kabul last week as their citizens were unable to reach the airport amid the deteriorating security situation.

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Britain still wants to fly thousands of people out from Afghanistan and has not set a hard deadline for when evacuations will end, Prime Minister Boris Johnson's spokesperson said on Monday.

Johnson will ask U.S. President Joe Biden to extend the evacuation deadline from Afghanistan when the Group of Seven leaders discuss the crisis on Tuesday.

According to a Reuters report, Germany is looking into ways to evacuate people from Afghanistan beyond the window of opportunity to fly them out of Kabul airport, a Foreign Ministry spokesman said on Monday.

"As long as the situation on the ground allows, we want to keep the air bridges active and evacuate people from Kabul airport," the spokesman told a regular government news conference in Berlin.

The US has warned its citizens to avoid Kabul airport amid concerns about the potential for attacks by Afghanistan's branch of the Islamic State (IS) group, according to BBC News. 

A security alert on Saturday told US citizens to stay away due to possible "security threats outside the gates. Only those individually told to make the journey by a US government representative should do so, it said.

The Taliban has agreed to provide “a safe passage” for civilians but this does not appear to be working on the ground.

An official said a "small number" of Americans and Afghans the US wanted to evacuate had faced harassment. In some cases, they had been beaten on their way to the airport.

NATO Secretary General Jens Stoltenberg admitted in an interview on CNN that at-risk Afghans were facing a problem in reaching the Kabul airport. He said that NATO was in touch with the Taliban leadership to allow such individuals to come to the airport.

The Taliban, on its part, has been defiant on the issue. "America, with all its power and facilities… has failed to bring order to the airport," Taliban official Amir Khan Mutaqi said.

"There is peace and calm all over the country, but there is chaos only at Kabul airport."