The US and British embassies in Kabul say the insurgent Taliban may have committed war crimes in southern Afghanistan by carrying out revenge murders of civilians, a charge denied by the insurgents.
Suhail Shaheen, a Taliban negotiating team member based in Doha, told Reuters on Monday tweets containing the accusations were "baseless reports".
The US mission tweeted a statement accusing the Taliban of killing dozens of civilians in the area of Spin Boldak in southern Kandahar province, the scene of heavy fighting. The statement was also tweeted by the British embassy.
"These murders could constitute war crimes; they must be investigated & those Taliban fighters or commanders responsible held accountable," the US embassy tweeted.
In a second tweet, it added: "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."
The tweets, accompanied by calls for a ceasefire, stepped up the United States' public criticism of the group as US troops withdraw and the Taliban presses a nationwide offensive.
US Secretary of State Antony Blinken, briefing journalists on a new US refugee program for US-affiliated Afghans, called reports of the Taliban committing atrocities "deeply disturbing and totally unacceptable".
State Department spokesman Ned Price said the Taliban are responsible for most of the "outrageous and atrocious acts of violence" in Afghanistan.
"The world will not accept a government in Afghanistan that doesn't respect basic human rights," he said, referring to the prospects of the Taliban reimposing its rule by force or joining an interim power-sharing arrangement as part of a peace deal.
The insurgents gained control last month of the strategic area of Spin Boldak, which lies at a border and trade crossing with Pakistan. Heavy fighting has taken place since as Afghan forces try to recapture the area.