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insurgents have seized 's second- and third-biggest cities, local officials said on Friday, as resistance from government forces crumbled and fears grew 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 U.S.-led international forces complete their withdrawal after 20 years of war.

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 the city of about 600,000 people near the border with Iran.

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

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

Washington on Thursday announced plans to send 3,000 additional troops to help evacuate USÌýEmbassy staff, and the Pentagon said most of the soldiers would be in Kabul by the end of the weekend. Britain also confirmed the start of a military operation to support the evacuation of its nationals.

"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 on Friday.

The United Nations has said it would not evacuate its staff from Afghanistan but was relocating some to Kabul from other parts of the country. Many Western embassies and aid groups said they were bringing many of their staff home, however.

UNÌýSecretary-General Antonio Guterres urged the Taliban to immediately halt their offensive.

"The message from the international community to those on the warpath must be clear: seizing power through military force is a losing proposition. That can only lead to prolonged civil war or to the complete isolation of Afghanistan," he told reporters.

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.

'Humanitarian Catastrophe'

The explosion in 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 start of the year, 250,000 of them since May, a UNÌýofficial said.

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

"The situation has all the hallmarks of a humanitarian catastrophe," the UNÌýWorld Food Programme's Thomson Phiri told a briefing.

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.

The Taliban has taken 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 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 and controlled most of the country from 1996 to 2001.

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 insurgents, was in their custody. Al-Jazeera later reported that Khan had boarded a plane to Kabul bearing a message from the Taliban. The report could not immediately be confirmed.

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 USÌýtroops.

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.

USÌýSecretary 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 USÌýSenate'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 Hanoi's victory in the Vietnam war. He urged Biden to commit to giving more support to Afghan forces.