A firefight outside Kabul's international airport killed an Afghan soldier, highlighting the perils of evacuation efforts even as the Taliban warned any attempt by US troops to delay their withdrawal to give people more time to flee would “provoke a reaction.”

The shooting came as the Taliban moved to shore up their position and eliminate pockets of armed resistance to their lightning takeover earlier this month.

The Taliban said they retook three districts north of the capital seized by opponents the day before and had surrounded Panjshir, the last province that remains out of their control.

Afghanistan's security forces collapsed in the face of the Taliban advance, despite 20 years of Western aid, training and assistance. Tens of thousands of Afghans have sought to flee the country since, fearing a return to the brutal rule the Taliban imposed the last time they ran Afghanistan.

US President Joe Biden has not ruled out extending the evacuation beyond August 31, the date he had set for completing the pullout of US forces. British Prime Minister Boris Johnson plans to press Biden for an extension.

But Taliban spokesman Suhail Shaheen, in an interview with Sky News, said August 31 is a “red line'' and that extending the American presence would “provoke a reaction.”

Gunfire broke out early Monday (local time) near an entrance to the airport, where at least seven Afghans died a day earlier in a panicked stampede of thousands of people.

William Urban, a US military spokesman, said an unknown assailant shot at Afghan security forces at the airport's northern gate, leading Afghan, US and allied troops to open fire in response. He said an Afghan soldier was killed and “several Afghans'' were wounded.

An Italian humanitarian organisation that operates hospitals in Afghanistan said it had treated six patients with bullet wounds from the airport.

There was no comment from the Taliban, who in recent days have fired warning shots and lashed out with batons to try to control crowds swelling into the thousands outside the airport.

The tragic scenes around the airport have transfixed the world. Afghans poured onto the tarmac last week and some clung to a US military transport plane as it took off, later plunging to their deaths. At least seven people died that day, in addition to the seven killed Sunday.

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The Taliban blame the chaotic evacuation on the US military and say there's no need for any Afghans to flee. They have pledged to bring peace and security after decades of war and say they won't seek revenge on those who worked with the US, NATO and the toppled Afghan government.

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

But Mohammad Khalid, another Taliban official addressing the same gathering, struck a more ominous tone, saying “history and Afghans will not forgive those who were trained in the US and Europe and returned to kill their own people.''

“So far, I can say that what we need is being granted; the danger comes more from these uncontrollable crowds of people.''

He said foreign countries should not interfere in education, asking the clerics if they would “tolerate a young girl sitting next to a boy at school.'' He also praised the role of suicide bombers in forcing the US to withdraw.

The divergent messages raised doubts as to whether the Taliban are fully united behind the more moderate image their leadership is projecting. There have also been reports in recent days of the Taliban hunting down their former enemies.

German Defence Minister Annegret Kramp-Karrenbauer told the Bild newspaper that the main obstacle to getting people out was the crowds outside the airport.

Asked about Taliban assurances of safe passage to the airport she said: “So far, I can say that what we need is being granted; the danger comes more from these uncontrollable crowds of people.''

As the airlift continues, the US government asked for 18 aircraft from American commercial carriers to assist in transporting Afghan refugees to their final destinations after their initial evacuation.

Since August 14, the US has evacuated or facilitated the evacuation of some 37,000 people on military and coalition flights. Those efforts are accelerating: In the 24 hours that ended early Monday, US military flights ferried about 10,400 people to safety, an official said.

Tens of thousands of people, Americans, other foreigners and Afghans who assisted in the war effort, are still waiting to join the airlift, which has been slowed by security issues and US bureaucracy hurdles.

There are also concerns that a local affiliate of the Islamic State group might target the crowds outside the airport with suicide bombers or fire missiles at US aircraft. Military planes have been executing corkscrew landings, and other aircraft have fired flares upon takeoff, both measures used to avoid missile attacks.

The Taliban and IS have different ideologies and have fought in recent years, but one concern about the Taliban's takeover is that they could again shelter extremist groups. The Taliban harboured al-Qaida while it orchestrated the 9/11 attacks, leading to the US invasion in 2001. The Taliban now say they will not allow Afghanistan to be a base for attacks on other countries.