NATO Secretary General Jens Stoltenberg said on Thursday allied forces should continue to evacuate as many vulnerable people as they can from Kabul despite what he branded a "horrific terrorist attack". 

"Our priority remains to evacuate as many people to safety as quickly as possible," he tweeted after two deadly explosions hit crowds on the perimeter of the airport in Afghanistan's capital. 

The attacks hit ahead of an August 31 deadline for US troops to leave the airport on the outskirts of the Afghan capital, and as allied militaries wind down their rescue operations.

Belgium and the Netherlands have already halted their airlift from the airport and other allied countries are to follow suit in the coming hours and days, despite fears that at-risk people will be left behind. 

The president of the European Commission, Charles Michel, echoed Stoltenberg's call for evacuations to continue.

"Securing safe passage to the airport remains vital," he tweeted.