South Sudan's army warned that government reports that a violent ethnic militia had disbanded appeared to be inaccurate.

"The forces of (former vice-president) Riek Machar are now advancing on Bor, but we are confident we will hold them off and protect the town," said army spokesman Philip Aguer.

It was not clear how many of the gunmen remained in the thick bush around Bor, but reconnaissance flights by the UN Mission in South Sudan (Unmiss) had identified armed groups, although observers could not confirm the force's size.