KABUL (Reuters) - Plans for the Afghan government to begin releasing Taliban prisoners were stalled on Saturday over disagreements on the number of detainees to be released and guarantees that they will not return to fighting, a government security spokesman said.

The Taliban promised to open talks with the Afghan government as part of an accord reached with the United States last month to end 18 years of U.S. involvement in war in Afghanistan.

The militants say the agreement requires the government to release 5,000 prisoners before talks begin. The government says the talks must begin and violence subside before it will free all the detainees.

Earlier this week President Ashraf Ghani agreed to free 1,500 prisoners on Saturday with plans to release the remainder once conditions were met.

"We are ready to start the process the way it is described in the presidential decree but we won't release anyone if there is no guarantee that they will not return to fighting," said Javid Faisal, spokesman for the Afghan government's national security advisor. "The Taliban have to show flexibility "

The Taliban are refusing to accept a partial release of the prisoners or any conditions on their release, its spokesman told Reuters on Friday.

"Our stance on the prisoners' release is crystal clear. They should be released without any pre-condition the way it is mentioned on the US-Taliban peace agreement," Zabihullah Mujahid told Reuters by phone. "We have not agreed to any extra dialogue or conditions."

Officials and experts say the issue poses a major hurdle to starting peace talks, alongside an additional challenge of rising discord between Ghani and his political rival Abdullah Abdullah.

Both men say they are the country's rightful leader. Their intense feuding is leading to the risk of parallel governments being formed and is hindering the appointment of a delegation to negotiate with the Taliban.