The rebel Liberation Tigers of Tamil Eelam (LTTE) accused the government of virtually declaring war on them with the latest offensive and vowed to resist a military advance to open a water canal blocked by the guerrillas 10 days ago in an area under their control.

The defence ministry said it had lost at least nine soldiers killed and six wounded, but managed to take control of the sluice gates by Monday afternoon after heavy mortar bomb exchanges with the Tigers. The ministry said "over 35" Tigers were killed.

The defence ministry said the offensive in the island's restive northeastern region of Trincomalee was aimed at ending the LTTE blockade of the Maavilaru waterway that had deprived thousands of farmers of water.

There was no immediate reaction from the Tigers to the casualty figures claimed by the military, but the guerrillas said they were resisting the military advance.

The LTTE also lodged a formal complaint with the Nordic truce monitors against the government, but made it clear they were not pulling out of the ceasefire agreement, a spokesman for the monitors said.

The Swedish-led Sri Lanka Monitoring Mission (SLMM) said the LTTE protested the military action.

"They (the LTTE) have said that the government is violating the ceasefire, but they have not made any decision to resign from the ceasefire agreement," spokesman Thorfinnur Omarsson told AFP.

Diplomats close to the peace process said the SLMM did not take seriously remarks by a regional Tiger leader asking them to declare that truce was not holding and that the fighting "tantamounted to a declaration of war."

Neither the Tigers nor the government have made any official statement on pulling out of the February 2002 ceasefire even though it is barely holding and each side has accused the other of violations that have been escalating since December.

The truce brokered by Norway can be terminated either by the Tigers or Colombo with a written declaration giving two weeks' notice.

The pro-rebel Tamilnet.com website said the Tigers resisted the military advance Monday.

"The Sri Lanka army began moving ground troops from two bases towards Maavilaru sluice gate Monday morning...," the Tamilnet said. "The Sri Lanka army troops were defeated by the Tigers."

Elsewhere, troops killed four Tigers in a mortar bomb attack in the northern peninsula of Jaffna, the defence ministry said in a statement. It said there were no casualties among troops in Jaffna.

Air strikes between Wednesday and Saturday killed at least 15 rebels but the government maintained it was not a return to full-scale war.

Security forces launched air strikes against Tiger positions shortly after a suicide assassination attempt against army chief Sarath Fonseka on April 25.

"Since end of April 2006... the Sri Lankan air force has dropped bombs in the Tamil homeland 13 times," the LTTE said Monday. "Thirty-three people were killed, 47 people were injured, and more than 7,000 people are displaced."

The violence erupted after presidential elections last November.

In a further threat to the truce, the LTTE has demanded that monitors from European Union members Finland, Denmark and Sweden leave the island after the EU added it to a list of "terrorist" organisations in May.

That would leave only Norwegian and Icelandic monitors.

Finland and Denmark announced on Friday they would pull out by the end of August. Sweden has yet to announce its position.

Norway is to send special envoy Jon Hansse-Bauer to Colombo next month to try to salvage the ceasefire, diplomats said.

More than 60,000 people have been killed in the three-decade-old Tamil separatist conflict.