South Sudanese troops are fighting to stop rebels loyal to former Vice President Riek Machar taking control of the major oil producing Upper Nile state capital, Malakal, the presidential spokesman said on Wednesday. "There has been heavy fighting between the forces loyal to Dr Riek Machar and the government forces (in Malakal)," presidential spokesman Ateny Wek Ateny told Reuters by phone. The rebels first attacked Malakal on Tuesday morning, he said. Ateny said Upper Nile oil fields, which produce about 200,000 barrels of oil per day, were far away from Malakal and under government control. "The oil fields are safe," he said.