Kabul: American troops have flown into Kabul to help evacuate embassy personnel and other civilians in the Afghan capital, a US 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 US 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.

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

Herat in the west, near the border with Iran, also fell to the militant group.

Kandahar's loss is a heavy blow to the government. It is the heartland of the Taliban ethnic Pashtun fighters who emerged in 1994 amid the chaos of civil war - and is close to the town of Spin Boldak, one of the two main entry points into Pakistan and a major source of tax revenues.

A US 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.

"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 US 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.

'Spinning out of control'

United Nations Secretary-General Antonio Guterres warned that "Afghanistan is spinning out of control" and urged all parties to do more to protect civilians.

"This is the moment to halt the offensive. This is the moment to start serious negotiation. This is the moment to avoid a prolonged civil war, or the isolation of Afghanistan," Guterres told reporters in New York.

Many people in the capital were stocking up on rice and other food as well as first aid, residents said. Visa applications at embassies were running in the tens of thousands, officials said.

Afghan First Vice President Amrullah Saleh said after a security meeting chaired by President Ashraf Ghani that he was proud of the armed forces and the government would do all it could to strengthen resistance to the Taliban.

The explosion in fighting has raised fears of a refugee crisis and a rollback of gains in human rights. Some 400,000 civilians have been forced from their homes this year, 250,000 of them since May, a UN official said.

Of Afghanistan's major cities, the government still holds Mazar-e-Sharif in the north and Jalalabad, near the Pakistani border in the east, in addition to Kabul.

The speed of the Taliban's gains has led to recriminations over the US withdrawal, which was negotiated last year under the administration of President Joe Biden's Republican predecessor, Donald Trump.

Biden said this week he did not regret his decision to follow through with the withdrawal. He noted Washington has spent more than $1 trillion and lost thousands of troops over two decades, and called on Afghanistan's army and leaders to step up.

Opinion polls showed most Americans back Biden's decision, but Republicans criticized the Democratic president's handling of the US withdrawal.

Senate Republican leader Mitch McConnell called the situation in Afghanistan "a debacle" but said it was not too late to stop the Taliban overrunning the capital by providing air and other support for Afghan forces.