By Louis Charbonneau

UNITED NATIONS, July 15 (Reuters) - U.N. Secretary-General Ban Ki-moon said in a new report that he was very worried that a Sudanese rebel group active in the country's war-ravaged Darfur region appeared to be using child soldiers.

In a bleak report on the joint United Nations-African Union peacekeeping mission in Darfur (UNAMID) released on Tuesday, Ban also said he was "deeply disappointed" by the lack of progress that has been made towards ending the 5-year-old conflict in western Sudan.

Ban said that a surprise attack on the Sudanese capital Khartoum in May by the rebel Justice and Equality Movement, which has generally confined its activities to Darfur, had highlighted the two sides' inability to settle their differences at the negotiating table.

Khartoum had said after the JEM attack that some of the rebels it captured after the attack were children. Ban made it clear that he found this to be a disturbing development.

"I am particularly concerned by reports of child soldiers among the JEM combatants in Omdurman, in clear violation of international law," Ban said.

"My office is pressing for the release of the children detained by the government (of Sudan) during the attack and I condemn the use of child soldiers in all instances."

Khartoum accuses Chad of backing JEM and has refused to negotiate with either JEM or the Chadians since the attack. Chad accuses the Sudanese government of backing rebels hoping to bring down its government.

Both countries reject the other's allegations.

"Tensions between Chad and the Sudan and the suspension of diplomatic relations between the two countries should also be highlighted as a source of considerable instability in Darfur and volatility on both sides of the border region," Ban said.

There are currently around 9,500 UNAMID troops and police in Darfur, well below the planned 26,000-strong force.

"NOT TOO LATE" FOR KHARTOUM

Ban's report, dated July 7, did not mention Monday's request by the International Criminal Court's chief prosecutor, Luis Moreno-Ocampo, for the ICC to issue an arrest warrant for Sudan's President Omar Hassan al-Bashir on suspicion of masterminding crimes against humanity.

Moreno-Ocampo accused Bashir of running a campaign of genocide that has killed 35,000 people outright, at least another 100,000 through a "slow death" and forced 2.5 million to flee their homes in Sudan's western region.

The French and U.S. envoys spoke to reporters about the ICC before a Security Council meeting on unrelated issues.

Permanent council member China has expressed concern about a possible ICC arrest warrant for Bashir and council diplomats say Beijing may ask for a council resolution demanding a 1-year suspension of any formal ICC indictment of Bashir.

French Ambassador Jean-Maurice Ripert said France believed the "Security Council should not mix up with the process, the due process of law." But he stopped short of saying Paris would veto any such resolution.

U.S. Ambassador Zalmay Khalilzad declined to comment when asked if Washington would veto a suspension resolution.

Ripert also said it was "not too late for the Sudanese authorities to cooperate" with the ICC by handing over to the court two men it indicted last year over Darfur -- Humanitarian Affairs Minister Ahmed Haroun and former Janjaweed militia commander Ali Kushayb.

The French envoy did not make clear, however, whether such a move might help Bashir escape charges.

If the ICC judges accept Moreno-Ocampo's recommendations, a warrant for Bashir could be expected in October or November. (Editing by Jackie Frank)

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