KABUL (REUTERS) - The Taleban pushed closer to Kabul on Saturday (Aug 14), capturing a key city near Afghanistan's capital as American troops flew in to help evacuate embassy personnel and other civilians.

Continuing a rapid advance, the insurgents took Pul-e-Alam, around 70km from Kabul and the capital of Logar province, a local provincial council member said.

The Taleban did not face much resistance, he told Reuters on condition of anonymity.

The gain of the city, a key staging post for a potential assault on Kabul, comes a day after the insurgents took 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 Taleban 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 hardline Islamist group.

Kandahar's loss was a heavy blow to the government. It is the heartland of the Taleban - 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 Taleban - ousted from power in 2001 after the Sept 11 attacks on the US - 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 Taleban 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," Mr 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 Taleban.

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 since the start of the year, 250,000 of them since May, a UN official said.

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

After seizing Herat, the insurgents detained veteran commander Ismail Khan, an official said. They had promised not to harm him and other captured officials.

A Taleban spokesman confirmed that Mr Khan, who had been leading fighters against the insurgents, was in their custody.

Al-Jazeera later reported Mr Khan had boarded a plane to Kabul bearing a message from the Taleban. The report could not immediately be confirmed.

The speed of the Taleban'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.

Mr Biden said this week he did not regret his decision to follow through with the withdrawal. He noted Washington spent more than US$1 trillion (S$1.35 trillion) and lost thousands of troops over 20 years of war, and called on Afghanistan's army and leaders to step up.

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

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