Taliban insurgents have seized Afghanistan's second- and third-biggest cities as resistance from government forces crumbled, stoking fears that an assault on the capital Kabul could be just days away.

A government official confirmed that Kandahar, the economic hub of the south, was under Taliban control as international forces complete their withdrawal after 20 years of war.

Herat in the west also fell to the hardline Islamist group.

"The city looks like a frontline, a ghost town," provincial council member Ghulam Habib Hashimi said by telephone from Herat, a city of about 600,000 people near the border with Iran.

"Families have either left or are hiding in their homes."

A senior US defence official said there was concern that the Taliban - ousted from power by USled forces in 2001 after the 9/11 attacks on the United States - could make a move on Kabul within days.

But Washington was hoping the Afghan security forces would put up more resistance as the insurgents move closer to the capital.

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 the resistance to the Taliban.

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

"The situation has all the hallmarks of a humanitarian catastrophe," the UN World Food Programme's Thomson Phiri told a briefing, expressing concern about a "larger tide of hunger".

Under Taliban rule, women could not work, girls were not allowed to attend school and women had to cover their face and be accompanied by a male relative if they wanted to venture out of their homes. In early July, Taliban fighters ordered nine women to stop working in a bank.

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.

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Sheltering in parks

Families were camping out in a Kabul park with little or no shelter, having escaped violence elsewhere in the country.

The United Nations has said a Taliban offensive reaching Kabul would have a "catastrophic impact on civilians". But there is little hope for a negotiated end to the fighting, with the insurgents apparently set on a military victory.

The Taliban also took the towns of Lashkar Gah in the south and Qala-e-Naw in the northwest, security officials said. Firuz Koh, capital of central Ghor province, was handed over without a fight, officials said.

The militants, fighting to defeat the government and impose their strict version of Islamic rule, have taken control of 14 of Afghanistan's 34 provincial capitals since Aug. 6.

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 Taliban spokesman confirmed that Khan, who had been leading fighters against the Taliban, was in their custody.

The Pentagon said on Thursday it would send about 3,000 extra troops within 48 hours to help evacuate US Embassy staff.

Britain said it would deploy about 600 troops to help its citizens leave. Many other embassies and aid groups said they were also getting their people out.

NATO allies held consultations on the crisis. Secretary-General Jens Stoltenberg tweeted: "Our aim remains to support the Afghan government & security forces. We maintain our diplomatic presence in Kabul & the security of our personnel is paramount."

The United Nations is evaluating the security situation on "an hour-by-hour basis" but is not evacuating staff from the country, spokesman Stephane Dujarric said. It is relocating some staff to Kabul from other parts of Afghanistan, he added.

Biden's decision

The speed of the Taliban offensive as US-led foreign forces prepare to complete their withdrawal by the end of this month has led to recriminations over President Joe Biden's decision to withdraw US troops.

Biden said this week he did not regret his decision, noting that Washington has spent more than $1 trillion in America's longest war and lost thousands of troops.

The loss of Kandahar 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.

US Secretary of State Antony Blinken and Defense Secretary Lloyd Austin told Ghani on Thursday the United States remained "invested" in Afghanistan's security.

But at home, criticism of Biden's policy has been mounting.

The US Senate's Republican leader Mitch McConnell, said the exit strategy was sending the United States "hurtling toward an even worse sequel to the humiliating fall of Saigon in 1975", referring to North Vietnam's victory in the Vietnam war. He urged Biden to commit to giving more support to Afghan forces.