The United States envoy negotiating with the Taliban voiced regret on Thursday over findings that US-backed forces were killing more civilians than the militants, and said the solution was a peace deal.

A United Nations report released on Wednesday found that international and pro-government forces were responsible for the deaths of 305 civilians in the first three months of the year ─ the first time they were behind more killings than the Taliban since the US-led coalition toppled the extremists' regime in 2001.

"We deeply regret any loss of innocent life during military operations. We never target innocents," said Zalmay Khalilzad, the US negotiator who is set shortly to resume talks with the Taliban in Qatar on ending the war.

"War is treacherous, and unintended consequences are devastating. While we strive to prevent casualties, real solution is a ceasefire or reduced violence as we pursue lasting peace," he tweeted.

Khalilzad appealed to the Taliban and other Afghans to "work to make this the year of peace".

He struck a different tone than the spokesman for US forces in Afghanistan, Colonel Dave Butler, who said the United States pursued "the highest standards of accuracy and accountability" and that troops "reserve the right of self-defence".

President Donald Trump is eager to find a negotiated way to pull out troops and end the longest-ever US war. Afghan President Ashraf Ghani, with whom the Taliban refuse to negotiate, has called for next week a loya jirga, a traditional gathering of all the country's communities, although it is unclear how broad the attendance will be.