By Roland Buerk
BBC News, Colombo
The Tamil Tigers are said to be putting up stiff resistance to the offensive
Sri Lankan soldiers have entered the strategically important Tamil Tiger stronghold of Pooneryn, according to the ministry of defence.
The move follows months of fighting and officials say pitched battles are still taking place in the area.
If the army succeeds in taking control, it will have access to the land route up the north-west coast to Jaffna.
The peninsula has been cut off by territory held by the rebels, who want a separate state for the Tamil people.
The Tamil Tigers have not yet commented on the reports.
Troops entered Pooneryn on Saturday morning, according to the defence ministry.
It said in a statement that soldiers had crossed marshlands south of the town during the night, and then marched into what was described as the rebels' last bastion on the western coast.
It added that the Tigers were putting up stiff resistance and that pitched battles were still going on in the area.
For months, amid heavy fighting, troops have pushed up the north-western coast towards Pooneryn, trying to open a land route to Jaffna, situated just across a lagoon from the town.
The far northern peninsula is in government hands, but has been cut off by territory held by the Tamil Tigers.
The only ways in and out for troops, supplies and civilians have been by sea and air.
The rebels, who have been fighting for decades for a separate state for the island's ethnic Tamil minority, have not commented on the defence ministry's claims.
The government has barred independent reporters from the areas where battles are taking place.
Army spokesman Brig Udaya Nanayakkara said government forces would now concentrate their thrust on the remaining territory held by the Tamil Tigers in the north-east of the island.
President Mahinda Rajapaksa has pledged to bring the rebels to their knees, unless they give up their arms and surrender.