JUBA, Sudan (Reuters) - Three Sudanese working with the Yemeni HTC oil company were killed and two Yemenis were believed missing after they were ambushed in Unity State in southern Sudan, a spokesman for the state said on Thursday.

Andrea Kuong told Reuters the group was ambushed while travelling between the Heglig oil-producing area in South Kordofan and Mayom County in Unity State on Wednesday.

"Two Yemenis were kidnapped, they (police) believe. But they are not 100 percent sure. They are still missing," he said.

He blamed the incident on the Baggara tribe, an Arab nomadic group that grazes cattle in Unity State. "This is not the first time Baggara have ambushed cars," he said.

The energy-producing region between north and south Sudan has seen escalating attacks against oil workers in the last six months.

Gunmen abducted nine Chinese oil workers from South Kordofan earlier this month and later killed four after what China said was a failed rescue attempt. Three workers escaped alive and two more remain missing. Sudan says the killings were unprovoked.

Khartoum blamed the kidnapping and killing of the Chinese workers on the Darfur rebel group the Justice and Equality Movement (JEM). The group has denied any involvement.

A 2005 peace deal that ended two decades of war between Khartoum and southern rebels formed the semi-autonomous Government of South Sudan, which has rights to 50 percent of the oil revenues generated in its territories.

Sudan produces around 500,000 barrels per day of crude.