* Darfur violence escalating * Sudan expelled the only aid agency working in area KHARTOUM, Feb 25 (Reuters) - The Sudanese army said it had attacked a Darfur rebel stronghold to open roads in the central Jabel Marra region and killed 25 rebels, the latest step in an escalation of fighting in western Sudan. U.N. humanitarian officials said some 13,000 new refugees had arrived this month at Zam Zam camp in North Darfur from areas which peacekeepers said had witnessed heavy clashes and government bombardment. President Omar Hassan al-Bashir has been indicted by the International Criminal Court for war crimes and genocide during Darfur's eight-year insurgency, but denies the charges. "The attack came in the context of armed forces' operations to open roads and secure villages at eastern Jabel Marra from ... rebels," army spokesman Al-Sawarmi Khaled said in a statement on the state news agency SUNA late on Thursday. He said the army had killed 25 rebel fighters and lost two of its own soldiers. The Darfur rebel Sudan Liberation Army (SLA) led by Abdel Wahed Mohamed el-Nur on Friday confirmed the attack to Reuters, but said the SLA had lost only six fighters and most of those killed were civilians. "They came with tanks and planes and they attacked all the villages in the area -- 13 villages are deserted and eight of those burned," said senior SLA commander Ibrahim el-Helu. Helu said the only international aid agency working in the area was the French agency Medecins du Monde, which the government expelled from South Darfur state earlier this month. "They were the only ones witnessing the crimes and they were the only ones working to help those in our rebel-controlled areas," he said, adding that was why the government expelled them. Government officials said the aid agency was helping the rebels and giving them information about government movements. Khartoum has been targeting foreign aid agencies for years, blaming them along with the Western media for exaggerating the extent of the Darfur conflict. Foreign aid groups have been arrested or expelled for talking about rape in Darfur. After the ICC issued its arrest warrant for Bashir in 2009, he expelled 13 of the largest aid agencies in the region. The aid effort has since struggled to fill the gaps and further expulsions and restrictions have created a code of silence among those still working in the region. Clashes in North Darfur state earlier this month between joint rebel forces and the government also forced several thousand people to flee to Zam Zam camp, U.N. humanitarian officials said. (Reporting by Opheera McDoom, editing by Tim Pearce)