JUBA (AFP) - A top South Sudan army commander based in the key oil-producing Unity state appears to have defected to rebel forces, the country's army spokesman said on Friday.

"We have lost contact with the commander... and there are reports he has joined the forces of Riek Machar," said army spokesman Philip Aguer, but he added that government forces were still in control of the area.

Fighting broke out last Sunday in the capital Juba between troops backing President Salva Kiir and those of Machar, his former vice president, but has since spread elsewhere in the country.

The spokesman said Major General James Koang Choul, commander of a division of the Sudan People's Liberation Army (SPLA) that controls Unity state, appeared to have defected alone.

"The members of the SPLA have not followed him, and we remain in control of Unity," Aguer said.

Aguer said shooting was reported overnight Friday to Saturday in Unity state's capital Bentiu, with oil companies evacuating workers.

The loss of the oil fields to rebels would be a major blow to the young nation.

Aguer said shooting was reported overnight Friday to Saturday in Unity state's capital Bentiu, with oil companies evacuating workers.

Chinese and Malaysian companies are among those who operate oil blocks in Unity state.