The US and Germany are telling their citizens in Afghanistan to avoid travelling to Kabul airport, citing security risks as thousands of desperate people gathered trying to flee almost a week after Taliban Islamists took control.

Mullah Baradar, the Taliban's co-founder, arrived in the Afghan capital for talks with other leaders on Saturday. The group is trying to hammer out a new government after its forces swept across the country as US-led forces pulled out, with the Western-backed government and military collapsing.

Crowds have grown at the airport in the heat and dust of the day over the past week, hindering operations as the US and other nations attempt to evacuate thousands of their diplomats and civilians as well as numerous Afghans. Mothers, fathers and children have pushed up against concrete blast walls in the crush as they seek to get a flight out.

The Taliban have urged those without travel documents to go home. At least 12 people have been killed in and around the single-runway airfield since Sunday when the Taliban seized control of Afghanistan, NATO and Taliban officials said.

"Because of potential security threats outside the gates at the Kabul airport, we are advising US citizens to avoid travelling to the airport and to avoid airport gates at this time unless you receive individual instructions from a US government representative to do so," a United States Embassy advisory said.

The German Embassy also advised its citizens not to go to the airport, warning in an email Taliban forces were conducting increasingly strict controls in its immediate vicinity. The advisories underscored just how unsettled the security situation remains.

A US official, speaking on condition of anonymity, said the United States military is looking at alternative routes for people to reach the airport because of threats from militant groups such as al Qaeda and the Islamic State.

Army Major General William Taylor, with the US military's Joint Staff, told a Pentagon briefing that 5800 US troops remain at the airport and that the facility "remains secure." Taylor said some gates into the airport were temporarily closed and reopened over the past day to facilitate a safe influx of evacuees.

A Taliban official, speaking to Reuters, said security risks could not be ruled out but the group was "aiming to improve the situation and provide a smooth exit" for people trying to leave over the weekend. The Taliban's takeover has sparked fear of reprisals and a return to a harsh version of Islamic law the Taliban exercised when they were in power two decades ago.

Taylor said the US in the past week had evacuated 17,000 people, including 2500 Americans, from Kabul. Taylor said in the past day 3800 people were evacuated on US military and chartered flights.

"Experts from the former government will be brought in for crisis management,"

Speaking a day after US President Joe Biden promised to evacuate "any American who wants to come home", Pentagon spokesman John Kirby said he did not have a "perfect figure" on how many United States citizens remain in Kabul and Afghanistan more broadly, though officials have indicated it is thousands.

Kirby declined to describe the specific "threat dynamics" in Kabul but called the security situation "fluid and dynamic".

"We're fighting against both time and space," Kirby said.

Switzerland postponed a charter flight from Kabul because of the chaos at the airport.

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"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," the Swiss Federal Department of Foreign Affairs said in a statement on Saturday.

Qatar's air force has evacuated Afghan nationals, students, foreign diplomats and journalists from Afghanistan, the Gulf country's government media office said on Twitter, giving no further details.

Crisis management

The Taliban official said the group plans to ready a new model for governing Afghanistan in the next few weeks, with separate teams to tackle internal security and financial issues.

"Experts from the former government will be brought in for crisis management," the official told Reuters.

The new government structure would not be a democracy by Western definitions, but the official said: "it will protect everyone's rights."

"We have heard of some cases of atrocities and crimes against civilians,"

Baradar will meet militant commanders, former government leaders and policymakers, as well as religious scholars among others, the official said.

The delay in forming a new government or even announcing who will lead a new Taliban administration underlines how unprepared the movement was for the sudden collapse of the Western-trained forces it had been fighting for years. The Taliban, whose overall leader Mullah Haibatullah Akhundzada has so far been silent publicly, must also unite disparate groups within the movement whose interests may not always coincide now that victory has been achieved. The Taliban follow an ultra-hardline version of Sunni Islam.

They have sought to present a more moderate face since returning to power, saying they want peace and will respect the rights of women within the framework of Islamic law.

When in power from 1996-2001, also guided by Islamic law, the Taliban stopped women from working or going out without wearing an all-enveloping burqa and stopped girls from going to school.

Individual Afghans and international aid and advocacy groups have reported harsh retaliation against protests, and round-ups of those who had formerly held government positions criticised the Taliban or worked with US-led forces.

"We have heard of some cases of atrocities and crimes against civilians," said the Taliban official, on condition of anonymity.

"If [members of the Taliban] are doing these law and order problems, they will be investigated," he said.