KHARTOUM (Reuters) - Darfur rebels said on Sunday they had shot down two Sudanese army helicopters in the latest reports of fighting that have marred faltering peace talks between Khartoum and other insurgents.

Sudan's army said two if its helicopters crashed after developing technical problems, but denied there was any attack.

"Government forces attacked our positions in South Darfur yesterday (Saturday)," Ibrahim al-Helwu, from the rebel Sudan Liberation Army (SLA) told Reuters, speaking by telephone from Paris.

"Our forces shot down two of their helicopters -- one crashed at the site and the other came down on its way back to Nyala (South Darfur's capital). The government says the war is over but this is just propaganda."

The SLA was one of two rebel groups that took up arms against Sudan's government in 2003, accusing Khartoum of neglecting the mostly desert western region.

The insurgents' Paris-based leader Abdel Wahed Mohamed al-Nur is refusing to negotiate with Khartoum, and dismissed a ceasefire signed last month between Sudan's government and Darfur's insurgent Justice and Equality Movement (JEM).

The SLA has reported regular army attacks on its positions in the central Jabel Marra region since Khartoum inked its deal with the militarily more powerful JEM in February. Aid groups say thousands have been displaced by clashes in the area.

Sudan's army spokesman Al-Sawarmi Khaled dismissed the rebel report, saying the SLA was the second insurgent force to try to take credit for the accident.

"The helicopters crashed because of technical problems. No person damaged them. Nobody targeted them," he told Reuters.

In a separate statement to state news agency Suna, Khaled said one helicopter developed a fault as it was flying low in the Shattaya area, between the towns of Kas and Nyala.

The engine of the second helicopter caught fire after sand got in as it tried to rescue the first crew, the statement said. No one was injured, Khaled added.

Further peace discussions between JEM and Sudan's government, hosted in the Qatari capital Doha, have stalled since the February ceasefire.

Sudanese government negotiator Amin Hassan Omer criticised JEM, telling journalists in Khartoum on Saturday it was not serious about the talks.

"The movement is not committed to the release of government prisoners ... It has breached the ceasefire it signed in Doha," he said.

JEM official Al-Tahir al-Feki told Reuters the discussions had reached "stalemate". The rebel group is insisting Sudan delay April presidential and legislative elections, saying they would be a farce while the conflict continues.

Khartoum has so far refused to budge on the vote date.