More than 1,000 Nigerian troops have arrived in Freetown, the capital of Sierra Leone, to reinforce an African peacekeeping force under renewed threat from rebels.

Their arrival came as heavy gunfire was heard on the outskirts of Freetown.

The information minister said the shelling was a pre-emptive measure by the intervention force.

Witnesses say rebels moved into the town of Waterloo, just outside Freetown, on Tuesday, before later withdrawing.

The rebels say their latest offensive is aimed at forcing the government of President Ahmad Tejan Kabbah to the negotiating table.

The government says it will only talk to the rebels if they lay down their arms.