American troops have flown into Kabul to help evacuate embassy personnel and other civilians in the Afghan capital, a U.S. official said on Saturday, a day after Taliban insurgents seized the country’s second- and third-biggest cities.

The Pentagon has said two battalions of Marines and one infantry battalion will arrive in Kabul by Sunday evening, involving about 3,000 troops.

They have arrived, their arrival will continue ’til tomorrow, the U.S. official told Reuters on condition of anonymity.

An infantry brigade combat team will also move out of Fort Bragg, North Carolina, to Kuwait to act as a quick reaction force for security in Kabul if needed, the Pentagon has said.

Britain and several other Western nations are also sending troops as resistance from Afghan government forces crumbles and fears grow that an assault on Kabul could be just days away, Reuters reported.

An Afghan government official confirmed on Friday that Kandahar, the economic hub of the south, was under Taliban control as U.S.-led international forces complete their withdrawal after 20 years of war.

A U.S. defence official said there was concern that the Taliban - ousted from power in 2001 after the Sept. 11 attacks on the United States - could make a move on Kabul within days, Reuters reported.

Kabul is not right now in an imminent threat environment, but clearly ... if you just look at what the Taliban has been doing, you can see that they are trying to isolate Kabul, Pentagon spokesman John Kirby said.

Some embassies have begun to burn sensitive material ahead of evacuating, diplomats said.

The U.S. embassy in the Afghan capital informed staff that burn bins and an incinerator were available to destroy material including papers and electronic devices to reduce the amount of sensitive material on the property, according to an advisory seen by Reuters.