COLOMBO, Sri Lanka - Sri Lankan troops backed by helicopter gunships attacked Tamil Tiger defenses in the northeast Thursday, a rebel-allied Web site reported, as international pressure grew for a new cease-fire to allow civilians to escape the fighting.

The government, which has surrounded the rebels along with tens of thousands of civilians in a sliver of land along the northeast coast, has vowed to crush the Tamil Tigers and end this Indian Ocean island nation's quarter century civil war.

The military initially denied launching new attacks on the rebels after its two-day unilateral cease-fire expired Wednesday, but the Ministry of Defense later posted a statement on its Web site announcing it had resumed the offensive and killed at least 19 rebels. There was no explanation for the government's conflicting reports.

Also Thursday, President Mahinda Rajapaksa visited the rebels' former administrative capital for the first time since the government captured Kilinochchi on Jan. 2, the government said in a statement.

Rajapaksa earlier this week announced the two-day "pause" in fighting to commemorate the Sri Lankan New Year and allow the civilians to flee the war zone. Only a few hundred crossed the front lines.

The remaining civilians are trapped in the crowded "no-fire zone," a civilian refuge inside rebel territory the government has promised not to attack. However, the military's rapid advance reportedly has pushed many Tamil Tiger fighters into the refuge and reports have grown of fighting in the area.

On Thursday morning, government troops, helicopters and artillery attacked fortifications erected by the rebels on the edge of the "no-fire zone," the rebel-affiliated TamilNet Web site said. The site, which reported that as many as 180 civilians were killed in fighting Wednesday, said it had no immediate details on casualties.

Meanwhile, foreign diplomats pressed for a new cease-fire.

British Foreign Secretary David Miliband and French Foreign Minister Bernard Kouchner issued a joint statement Wednesday accusing the rebels of forcing the civilians to stay in the war zone for use as human shields against the government offensive.

The statement called on Rajapaksa to again halt the fighting.

"It is vital that a pause in the fighting should be long enough to give civilians the opportunity to leave the conflict area, and for the U.N. to build confidence amongst the population that they will be safe if they leave," the statement said.

The government has brushed off calls for a new cease-fire.

John Holmes, the U.N. humanitarian chief, also urged a longer halt to the fighting to get the civilians out and avoid "a bloodbath on the beaches." The rebels were clearly preventing the 100,000 trapped civilians from escaping, he said, adding that he contacted rebels outside the war zone in recent weeks to say it was unacceptable to "hold people against their will."

The rebels say the civilians do not want to leave.

In recent months, the government has forced the rebels out of much of the de facto state they ran in northern Sri Lanka.

The rebels have been fighting for 25 years to create an independent homeland for ethnic minority Tamils, who have faced decades of marginalization by successive governments controlled by the ethnic Sinhalese majority. More than 70,000 people have been killed in the violence.

Holmes called reports that the casualty toll in the war zone was rising once again "very worrying," and urged the government to live up to its promises not to use heavy weapons in the area.

"I'm afraid they have been doing that in some respects, and that's one of the factors which is obviously causing these civilian casualties," he said, adding that "dozens a day are killed."

Associated Press writers Edith M. Lederer at the United Nations and David Stringer in London contributed to this report.

A service of the Associated Press(AP)