The battle for the Libyan oil town of Brega switched from the desert to intense street fighting, as veteran leader Muamer Gaddafi vowed never to quit and fresh blasts rocked Tripoli.

Rebel forces re-entered Brega -- putting them within sight of a major strategic victory -- but said they had not yet managed to wrest control of the town from Gaddafi's troops, who have held it since April.

"Some small groups have made it inside, but we do not control the whole (town) yet," said Mohammed Zawi, a spokesman for the rebel forces, late Sunday.

He also dismissed rumours that Gaddafi’s troops had abandoned the town altogether. "It is now close fighting," he said, indicating a new phase in the four-day rebel campaign. There was no way to verify his comments as reporters are being prevented from reaching the front line.

Until now heavy artillery had set the tenor of the battle, but mortars and rockets appear to have given way to heavy machine guns -- a more useful weapon for fighting at closer quarters.

But that did little to stem the bloodshed.

Three rebel fighters were killed and 96 wounded on Sunday according to medical staff.

That toll brought to 15 the total number of fighters killed and 274 the number of wounded since the battle for Brega began on Thursday.

In Brussels, a NATO statement said its forces were monitoring "the dynamic situation across the country, including around Brega."