KHARTOUM, June 1 (Reuters) - A Sudanese security official said on Tuesday rebels in west Sudan were repelled when they attacked a strategic town but a rebel gropup denied there had been an offensive because the town was already in their hands.

The government and rebels in Darfur have both accused the other of violating a ceasefire deal reached in April. The latest reported violence comes on the day the first batch of African Union (AU) truce monitors were due to arrive in the country.

Although the government has moved close to a final deal on ending a conflict in the south of the oil-producing country, violence has continued to rage in the west, creating what the United Nations says is the world's worst humanitarian crisis. The security official told Reuters rebels launched an attack on Tuesday morning on Ain Sirou, a town north of Kebkabiya which in turn sits on a major transport route running east to west through the Darfur region.

"(The attack) is the last in a series of attacks that have been ongoing in the last few days...They (the rebels) were repelled by our forces and they regrouped and launched a second attack," he said.

The official said fighting was continuing late on Tuesday afternoon. He added he did not have casualty figures.

"There have been a series of violations of the ceasefire by the rebels and we are expecting more," he said.

"Ain Sirou is hugely important. It is what we consider militarily to be a strategic area. If taken, attacks could be launched from it to Al- Fashir and other important areas," he said, referring to the capital of Northern Darfur state which is about 140 km (85 miles) east of Kebkabiya.

But a spokesman for the Sudan Liberation Army (SLA), one of the two main rebel groups which took up arms against the government in February 2003, dismissed the report.

"How can we attack? It is already in our hands," the SLA's Mohammed Mursal said by telephone from Geneva, where he was due to hold talks on aid issues with U.N. and government officials.

"Furthermore, nothing is happening there. I was just speaking to our commanders in the field," he added.

Mohamed Yousif Abdallah, Sudanese minister of state for humanitarian affairs, said AU ceasefire observers were due to arrive on Tuesday. "Preparations have been made for them in Al-Fashir," he said.

The AU said on Friday the first seven observers would go to Darfur this week. It has said it was planning to send a mission of 60 military officials and about 30 civilians to five flashpoints in Darfur to monitor ceasefire violations.

Witnesses told Reuters on Friday that 12 people were killed when an Antonov plane and helicopters bombed the village of Tabit, about 40 km (25 miles) southwest of Al-Fashir.