Rebels in South Sudan have launched a large-scale attack on the strategic northern town of Malakal, an official and aid sources said Saturday.

Fierce fighting was reported to be raging inside the town, situated in the country's oil-rich north, in what appeared to be a major counter-attack against a several weeks-old offensive by government troops.

"The rebels of Riek Machar have attacked Malakal from all directions, from east, west, north and south, and the fighting up to now is continuing," Information Minister Michael Makuei told AFP.

He said government troops had so far "managed to repulse the rebels" to keep them from capturing the town, the capital of Upper Nile State.

Officials said the attack began just before darkness on Friday, with rebels crossing the White Nile river on boats.

Makuei said fighters loyal to rebel leader Macher, who have been fighting to overthrow President Salva Kiir since December 2013, were aided by a local militia commander from the ethnic Shilluk people who had been a government general.

The commander, Johnson Olony, is a powerful commander in the key oil-producing state of Upper Nile. He has also been accused of abducting scores of children to fight in his force.

"It was Olony who transported the rebels across the river bank to Malakal," Makuei said.

Aid workers in the town who spoke to AFP reported intense gunfire as well as the thump of heavy explosions from artillery or mortar fire.

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