WRAPUP 6-Firefight involving Western forces erupts amid Kabul airport evacuation chaos

(Adds comment from British PM's team)

* Afghan soldier killed in clash with unidentified assailants

* Biden says evacuation 'hard, painful', thousands getting out

* Taliban says no extension to Aug. 31 deadline sought yet

* WHO can't send medical supplies because of airport disruption

KABUL, Aug 23 (Reuters) - A firefight involving Western forces erupted at Kabul airport on Monday when Afghan guards exchanged fire with unidentified gunmen, Germany's military said, adding to the evacuation chaos as Washington faces pressure to extend its deadline to withdraw.

Thousands of Afghans and foreigners have thronged the airport for days, hoping to catch a flight out after Taliban fighters captured Kabul on Aug. 15 and as U.S.-led forces aim to complete their pullout by the end of the month.

Twenty people have been killed in the chaos at the airport, most in shootings and stampedes in the heat and dust, penned in by concrete blast walls, as U.S. and international forces try to evacuate their citizens and vulnerable Afghans. One person was killed in Monday's clash, the German military said.

CNN said a sniper outside the airport fired at Afghan guards - some 600 https://www.reuters.com/world/asia-pacific/hundreds-afghan-security-forces-help-kabul-airport-pentagon-says-2021-08-17 former government soldiers are helping U.S. forces at the airport - near its north gate.

U.S. and German forces were involved in the clash, Germany's military said. Three wounded Afghan guards were being treated at a field hospital in the airport, it said.

Two NATO officials at the airport said the situation was under control after the firing.

The Taliban have deployed fighters outside the airport, where they have tried to help enforce some kind of order.

On Sunday, Taliban fighters beat back crowds at the airport a day after seven Afghans were killed https://www.reuters.com/world/asia-pacific/seven-people-killed-crowd-near-kabul-airport-uk-ministry-defence-2021-08-22 in a crush at the gates as the deadline for the withdrawal of foreign troops approaches.

The Taliban seized power just over a week ago as the United States and its allies withdraw troops after a 20-year war launched in the weeks after the Sept. 11, 2001, attacks as U.S. forces hunted al Qaeda leaders and sought to punish their Taliban hosts.

The administration of Biden's predecessor, Donald Trump, struck a deal with the Taliban last year allowing the United Sates to withdraw its forces in exchange for Taliban security guarantees.

'HOURS, NOT WEEKS'

President Joe Biden said on Sunday the security situation in Afghanistan was changing rapidly and remained dangerous.