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.

US defence officials said they were monitoring developments and looking at alternative routes.

The US advice on Saturday came amid increasing chaos outside Kabul airport as thousands of Afghans are trying to flee the country as they fear a reprisal from the Taliban.

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Other countries have also issued warnings over the deteriorating security situation around the airport. Switzerland and Germany have cancelled evacuation flights from Kabul as people are not being able to reach the airport due to the worsening security situation in Afghanistan.

"The security situation around Kabul airport has worsened significantly in the last hours. A large number of people in front of the airport and sometimes violent confrontations are hindering access to the airport in Kabul," Switzerland’s Federal Department of Foreign Affairs (FDFA) said in a statement.

Germany's government issued a statement saying the airport remains "extremely dangerous and access to the airport is often not possible."

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

India’s embassy staff in Kabul had also been held up for a day as they could not reach the Kabul airport on Monday but eventually contact was established with the Taliban leadership to enable them to board the Indian Air Force plane at the Hamid Karzai Airport on Tuesday. Armed Taliban fighters had escorted the Indian contingent to the airport.

Sky News' chief correspondent Stuart Ramsay said on Saturday that people at the front of the crowd of thousands were being "crushed to death", with British soldiers pulling those in danger from the crowd.

He has described it as "the worst day by far", and said they believed people had died at the scene.

In a briefing on Saturday, the US Department of Defense said 17,000 people had been flown out of the airport, including some 2,500 US citizens.

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.