The rebels are fighting a rearguard action in the north
Sri Lankan soldiers have captured the first line of defence of Tamil Tiger rebels on the northern Jaffna peninsula, the army has said.
A military spokesman said 50 rebels and about 10 soldiers had been killed during several days of heavy fighting.
The Sri Lankan army overran a five-mile (8km) embankment dotted with fortified bunkers, he said.
The rebels are said to have three more defensive lines on the land dividing their territory from the government's.
Government forces are seeking to crush the Tigers and end their fight for a separate state for the ethnic Tamil minority.
In recent months the rebels have been driven from many towns and villages along the coast in the north-west.
Sri Lankan security forces are also close to the rebels' administrative headquarters in the northern town of Kilinochchi.
The first line of Tamil Tiger defences at Muhamalai fell after several days of heavy fighting, Sri Lanka's military spokesman Brigadier Udaya Nanayakkara said.
The Tigers are believed to have three more defensive lines on the narrow isthmus of land that divides rebel territory from the government-controlled Jaffna in the far north, he said.
The Tigers have not commented on the military's claims and the government strictly controls access by journalists to the area where the fighting is taking place.
The BBC's Roland Buerk in Colombo says Sri Lanka's military is on a major offensive against the rebels and troops have been advancing rapidly.
But there is concern over the fate of hundreds of thousands of civilians who have fled their homes and are now trapped in the diminishing rebel territory, our correspondent says.
Amnesty International has said the government is incapable of providing for their basic needs, such as shelters, sanitation and safe drinking water.
The human rights organisation has also accused the Tigers of not allowing civilians to move to safer places and of using them as a human shield against the military's onslaught.
Earlier this week, Sri Lankan troops captured a strategically important town used by the Tigers as a key supply route to the island's north, the army said.
It was the first time in recent fighting that the army had captured parts of the A9 highway leading from the centre of the island to the Jaffna peninsula, correspondents said.
Brig Nanayakkara said the army had taken control of Mankulam town, near the A9, on Monday.
He said that in capturing the town, in rebel hands since 1999, the army had cut off rebel supply routes.
On Sunday, the army said it had entered the Tamil Tiger stronghold of Pooneryn and now controlled the entire western coast.
The rebels have been fighting for a separate homeland for Tamils in the north and east since 1983 and about 70,000 people have been killed in the violence.