Sri Lankan troops are now attempting to protect their territorial gains
The Sri Lankan army says it has seen its heaviest casualties in a single day since the government pulled out of a ceasefire with Tamil rebels in January.
At least 52 Tamil Tiger rebels and 38 soldiers were killed in fighting in the Jaffna peninsula in the far north of the country, a military spokesman said.
But the Tigers say they killed more than 100 troops and lost 16 of their own, beating back a major offensive.
There has been no independent confirmation of the casualty figures.
Fighting in Sri Lanka has intensified since the government pulled out of the 2002 ceasefire deal, saying the rebels had used it to rearm and regroup.
'Front line crossed'
More than 80 soldiers were wounded in the latest heavy fighting, which broke out along the front line in the Muhamalai area on Wednesday morning, Sri Lankan Army (SLA) spokesman Brig Udaya Nanayakkara told the BBC.
Soldiers from two SLA divisions crossed the front line dividing territory held by the Tigers from government-controlled areas in the peninsula, after the rebel group attacked army positions, he added.
Brig Nanayakkara said the troops were able to advance about 500m (1,650ft) along an 8km-wide front, overrunning the rebel's first line of defence.
The Tigers' military spokesman, Rasiah Ilanthiraiyan, accused the army of sparking the battle.
"They attempted to get near our positions. That's when the clashes erupted," he told the Associated Press.
The pro-rebel TamilNet website quoted the group's Northern Forces Operations Command as saying the army had been "forced to retreat, leaving behind dead bodies of their troopers" after the battle ended.
"Special commando units of the Tigers were also engaged to confront the SLA units that attempted to advance using Main Battle Tanks amid heavy Multi-Barrel Rocket Launcher (MBRL), artillery and mortar fire," the website reported.
On Tuesday, the Tigers said they had destroyed a tank during battles with the army in the same area. The military said 10 rebels and one soldier had died in the clashes.
Correspondents say both sides frequently exaggerate enemy losses and play down their own.
The Tigers have fought for a generation for an independent state for the Tamil minority. At least 70,000 people have died since the civil war began in 1983.