Afghan forces battled to stop the first major city from falling to the Taliban as the United States and Britain accused the insurgents of "massacring civilians" in a town they recently captured near the Pakistani border.

Taliban fighters assaulted at least three provincial capitals overnight -- Lashkar Gah, Kandahar and Herat -- after a weekend of heavy fighting that saw thousands of civilians flee the advancing militants.

Fighting raged in Helmand's provincial capital Lashkar Gah, where the Taliban launched coordinated attacks on the city centre and its prison -- just hours after the government announced the deployment of hundreds of commandos to the area.

The war has intensified since early May, with the insurgents capitalising on the final stages of the withdrawal of US-led foreign forces after almost 20 years.

President Ashraf Ghani blamed Washington for deteriorating security.

"The reason for our current situation is that the decision was taken abruptly," Ghani told parliament, referring to the withdrawal of foreign forces.

Ghani said he had warned Washington that the withdrawal would have "consequences".

His comments came as Washington said that, in light of the increased violence, it will take in thousands more Afghan refugees including those who worked with the United States.

Washington has already started evacuating thousands of interpreters and their families who worked with the US military and embassy over the past two decades.

- 'Disturbing and unacceptable' -

The United States and Britain on Monday accused the Taliban of atrocities that may amount to "war crimes" in the town of Spin Boldak, which the insurgents captured last month along the border with Pakistan.

The diplomatic lashing comes after Afghanistan's Independent Human Rights Commission said the insurgents had indulged in revenge killings in Spin Boldak.

"The Taliban chased and identified past and present government officials and killed these people who had no combat role in the conflict," the group said, adding at least 40 people had been killed by the Taliban.

"These murders could constitute war crimes," the embassies of Washington and London said in separate tweets.

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Taliban leadership must be held responsible, they said, adding: "If you cannot control your fighters now, you have no business in governance later."

Top US diplomat Antony Blinken also slammed the militant leaders, saying the reports were "deeply disturbing and totally unacceptable."

An Afghanistan without a democratic, inclusive government would be a "pariah state," he said, adding that the international recognition the group wants will not be possible if "seeks to take the country by force and commits the kind of atrocities that have been reported."

State Department spokesman Ned Price said the Taliban attacks show "little regard for human life."

"If the Taliban leadership truly supports a negotiated solution to this conflict, as they say they do... they must stop these horrific attacks," he said. 

- 'Many are suffering' -

Fighting meanwhile continued in the Lashkar Gah overnight as Afghan forces beat back a fresh Taliban assault.

Resident Hawa Malalai warned of a growing crisis in the southern city: "There is fighting, power cuts, sick people in hospital, the telecommunication networks are down. There are no medicines and pharmacies are closed."

Medical charity Doctors Without Border said casualties were mounting in Lashkar Gah.

"There has been relentless gunfire, airstrikes and mortars in densely populated areas. Houses are being bombed, and many people are suffering severe injuries," said Sarah Leahy, the aid group's coordinator for Helmand, in a statement.

Helmand for years was the centrepiece of the US and British military campaign in Afghanistan. The province's vast poppy fields provide opium for the international heroin trade, a lucrative cash source for the Taliban.

The loss of Lashkar Gah would be a massive strategic and psychological blow for the government, which has pledged to defend cities at all costs after losing much of the rural countryside to the Taliban over the summer.

Fighting also surged in some districts of Kandahar province, the former bastion of the insurgents, and on the outskirts of its capital.

In the west, hundreds of commandos were also defending Herat after days of fierce fighting.

"If Afghan cities fall... the US decision to withdraw from Afghanistan will be remembered as one of the most notable strategic blunders in American foreign policy," Australia-based Afghanistan expert Nishank Motwani told AFP.