An unidentified diplomatic source told the Reuters news agency: "One driver was released and handed over a note by the captors demanding a settlement through a share of oil production."

The source said that members of the Arab Messeria tribe had carried out the attack because they want a greater share of the region's oil revenue.

Li Chengwen, the Chinese ambassador to Sudan, said: "We are doing our best efforts to find them."

It is the third time within the last year that oil workers have been kidnapped in the region, the source of a large part of Sudan's oil wealth.

Officials say Sudanese forces have been deployed and are searching for the men, but no contact had been made with the kidnappers.