US negotiator Zalmay Khalilzad has returned to Qatar for talks with Taliban: State Department

US negotiator Zalmay Khalilzad has returned to Qatar for further talks with the Taliban as he fine-tunes a deal on withdrawing troops, the State Department said on Thursday.

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

The Taliban, however, 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.

On Twitter, Taliban spokesman Zabihullah Mujahid claimed responsibility for the attack, saying a "martyrdom seeker" - or suicide bomber - had triggered the car bomb and that 12 "foreign invaders" were killed.