The rebel Liberation Tigers of Tamil Eelam (LTTE) used 122-millimeter artillery to target the base at Trincomalee as a ferry carrying 800 troops was about to dock there, military officials said.

"The 'Jetliner' ferry was making its way to the harbour when the... attack started," a military official said.

He said five suspected rebel boats pursued the ferry transporting troops returning home on leave, but the navy was able to beat them back while ground troops fired multi-barrel rocket launchers to neutralise rebel positions.

"Four naval ratings were killed and about 30 injured," the military official said. "Several buildings caught fire and we are putting them out."

At least 36 explosions were heard from within the naval compound next to the main Trincomalee harbour and also damaged a navy gunboat, official sources said, adding that initially they believed the Tigers were firing mortar bombs.

Warplanes swung into action for the second time Tuesday following the attack on the base, after attacking Tiger positions earlier in the day in the same region, the defence ministry said.

Officials said Israeli-built Kfir jets had carried out bombing sorties around dawn Tuesday near the Maavilaru irrigation canal in Trincomalee district, which the rebels blocked 10 days ago, sparking bitter fighting.

The ground offensive against the Tigers was at its bloodiest on Monday claiming at least 67 lives on both sides and making a mockery of a ceasefire in force since February 2002.

"We can't say how long it will take for us to open the sluice gates," government spokesman Keheliya Rambukwella told reporters. "We have to move very cautiously."

The military has said it suspects the area is heavily mined and booby-trapped.

Rambukwella said troops were engaged in "consolidation" Tuesday while continuing long-range artillery attacks against Tiger targets.