The London Embassy and Washington Embassy said in tweets on Monday: “The Taliban massacred dozens of civilians in revenge killings. These murders could constitute war crimes.

“The Taliban’s leadership must be held responsible for the crimes of their fighters. If you cannot control your fighters now, you have no business in governance later.”

Meanwhile, speaking to the BBC, General Sami Sadat, who is leading the fight against the Taliban in Helmand province, the scene of intense fighting in recent days, said the crisis had implications far beyond his country’s borders.

He sad the Taliban was being reinforced by other groups from outside the country.

He added: “This will increase the hope for small extremist groups to mobilise in the cities of Europe and America, and will have a devastating effect on global security.

“This is not a war of Afghanistan, this is a war between liberty and totalitarianism.”

Afghanistan’s Independent Human Rights Commission has said Taliban fighters carried out revenge killings in Spin Boldak too, which has further outraged US and UK relations.

Thousands of civilians have already had to flee from Lashkar Gah, Kandahar and Herat just before Taliban fighters attacked and continued a weekend of destruction on the three provincial capitals.

The Taliban said: “After taking over Spin Boldak district, the Taliban chased and identified past and present government officials and killed these people who had no combat role in the conflict.”

Sarah Leahy, the Medicine Sans Frontiers Helmand co-ordinator said: “There has been relentless gunfire, air strikes and mortars in densely populated areas. Houses are being bombed, and many people are suffering severe injuries.”

The US has offered to take in Afghan refugees who are fleeing the fighting.

The US State Department said on Monday: “In light of increased levels of Taliban violence, the US government is working to provide certain Afghans, including those who worked with the United States, the opportunity for refugee resettlement.”

Ashraf Ghani the President of Afgahanistan said: “The reason for our current situation is that the decision was taken abruptly.”

He also warned Washington their withdrawal would have “consequences”.

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