Taliban militants will not take part in intra-Afghan talks until about 5,000 of their prisoners are released, a spokesman said on Monday, presenting a major possible barrier to ending the war.

Under an accord between the United States and the Taliban signed on Saturday, the two sides are committed to working towards the release of combat and political prisoners as a confidence-building measure.

The agreement calls for up to 5,000 jailed Taliban prisoners to be released in exchange for up to 1,000 Afghan government captives by March 10.

Afghan President Ashraf Ghani, not involved in the talks, has since rejected that demand.

“We are fully ready for the intra-Afghan talks, but we are waiting for the release of our 5,000 prisoners,” Taliban spokesman Zabihullah Mujahid told Reuters by phone.

“If our 5,000 prisoners — 100 or 200 more or less does not matter — do not get released there will be no intra-Afghan talks.”

The US has said it hopes negotiations towards a permanent political settlement and ceasefire can start in coming days, but Western diplomats and analysts see stark challenges ahead.

Ghani said on Sunday that US President Donald Trump had not asked for the release of the prisoners and that the issue of prisoner releases should be discussed as part of a comprehensive peace deal.

Zabihullah said the majority of prisoners on the list of 5,000 had been captured by American forces and were held in Afghan government prisons and that they had prioritised sick and older prisoners. US-led forces ousted the hardline militants from power in 2001.

Zabihullah said that an agreement of a reduction in violence in the seven days leading up to Saturday's pact in Doha had formally ended.

“As we are receiving reports that people are enjoying the reduction in violence, we don't want to spoil their happiness, but it does not mean that we will not take our normal military activities back to the level that we were before,” he said.

“It could be any time, it could be after an hour, tonight, tomorrow or the day after.”

The Afghan war has been a stalemate for more than 18 years, with Taliban forces controlling or contesting more territory yet unable to capture and hold major urban centres.

Under the agreement, the US is committed to reducing the number of its troops in Afghanistan to 8,600 from 13,000 within 135 days of signing.

It also is committed to work with allies to proportionally reduce the number of coalition forces in Afghanistan over that period, if the Taliban adhere to their security guarantees and ceasefire.

A full withdrawal of all US and coalition forces would occur within 14 months, a joint statement said. The withdrawal depends on security guarantees by the Taliban.