The battlefield south of Kilinochchi is a mud bath (Photo: Ministry of Defence)
At least 33 troops were killed fighting Tamil rebels in northern Sri Lanka over the weekend, the government says.
The Tamil Tigers also suffered heavy losses, the defence ministry said. There has been no word from the rebels.
The military claims to have breached a key rebel defensive line near the rebels' administrative headquarters in the northern town of Kilinochchi.
The army's losses are among the highest it has admitted suffering in fighting to capture the rebel-held town.
The army is pursuing an offensive to defeat the rebels and end their fight for independence for minority Tamils.
The Ministry of Defence said troops and rebels fought battles in several areas to the south and west of Kilinochchi on Saturday and Sunday.
"During these clashes on the weekend, 33 soldiers were reported killed in action, 48 injured and three others were reported missing," a statement on the ministry's website said.
It said ground and radio monitoring sources had reported "heavy damages" to the rebels. The bodies of 11 Tamil Tiger fighters had been recovered by troops, the statement said.
There was also heavy fighting around a Tiger naval base to the west of Kilinochchi.
The BBC's Roland Buerk in Colombo says it is rare for the military to admit taking such heavy casualties.
A long earth embankment at Vannerikulam to the south of Kilinochchi - the last major defensive line protecting the town - was captured during the fighting, according to the ministry.
Our correspondent says the military offensive could be slowed by the monsoon rains that have now begun in the north of Sri Lanka.
Photographs from the battlefield posted on the ministry of defence website showed vehicles bogged down in heavy mud.
The ministry also claimed the Tigers had used poisonous gas during the battles.
The rebels could not be reached for comment and the government blocks independent journalists from going to the areas where the fighting is taking place.
Fears for civilians
Capturing Kilinochchi would be a major symbolic victory for Sri Lanka's government - the rebels have run a civil administration from offices in the town.
But many residents and rebels are reported to have abandoned it for areas further east still under Tamil Tiger control.
Much of the Tigers' military strength is concentrated to the east of the town, towards Mullaitivu on the coast.
Meanwhile, Sri Lanka's President Mahinda Rajapaksa has told Indian Prime Minister Manmohan Singh that the military has been instructed to avoid causing civilian casualties.
There have been protests by some Tamils in southern India against the Sri Lankan military offensive.
Last week, the Indian government said a military victory would not end the ethnic conflict and called for a political settlement.
The rebels have been fighting for a separate homeland for a quarter of a century and about 70,000 people have been killed in the violence.