"Ambassador Khalilzad has returned to Doha to continue talks," a State Department spokesperson said.

Khalilzad had earlier reached an agreement "in principle" with the Taliban that would lead to an end to America's longest war, a top goal for President Donald Trump.

The internationally recognized government in Kabul, with which the Taliban have refused to negotiate, voiced concern and sought clarifications, pointing to risks.

The Taliban have not relented on violence despite negotiating with the United States, killing at least 10 people including NATO troops in a bombing Thursday in Kabul.