Government troops launched an offensive Wednesday against rebel fortifications in northern Sri Lankan, destroying 30 bunkers and killing 12 Tamil Tiger fighters, while air force jets bombed a meeting of the separatist group's leaders, the military said.

The attacks were part of an escalating operation against the rebels' de facto state in the north. President Mahinda Rajapaksa's government has promised to destroy the group by the end of the year and end the long-running war in this Indian Ocean island nation.

In the fighting Wednesday morning, troops backed by artillery, mortar fire and tanks crossed over the front lines south of rebel-held territory about 5:15 a.m. and attacked a string of Tamil Tiger bunkers, said military spokesman Brig. Udaya Nanayakkara.

After about an hour of fighting that killed 12 rebels, the troops returned to their positions, he said.

"We don't want (the Tamil Tigers) to get organized in the first line of defense so they would start troubling us," he said. "We wanted to give them maximum damage and maximum destruction."

Two hours later, air force jets bombed a meeting of rebel leaders northwest of Kilinochchi, the main town controlled by the Tamil Tigers, Nanayakkara said.

Rebel spokesman Rasiah Ilanthirayan did not answer repeated calls from The Associated Press seeking comment, however the Tamil Tigers and the military routinely give vastly different accounts of the same battles.

With the fighting increasing in the north, bombings of buses, train stations and other targets deep inside government-controlled territory have escalated, with the military blaming the rebels for the attacks.

Suspected rebels detonated a bomb at a power transformer in a Colombo suburb early Wednesday, and a bomb at a second transformer was found late Tuesday, Nanayakkara said.

Since Saturday, at least 43 people were killed in two separate bus bombings and a bombing attack on Colombo's main railway station.

The Tamil Tigers have been fighting since 1983 for an independent homeland for ethnic minority Tamils after decades of being marginalized by Sinhalese-dominated governments. More than 70,000 people have died in the fighting.

Amnesty International said in a statement Tuesday that the government and the rebels were "failing to comply with their obligations under international humanitarian law and are killing civilians on an increasingly regular basis."

"With no perpetrators brought to justice, a climate of impunity is becoming entrenched: Unless these patterns are reversed the future appears bleak," the statement quoted Tim Parritt, deputy program director for Asia-Pacific, as saying.