Sri Lankan soldiers have entered the strategically important Tamil Tiger stronghold of Pooneryn, according to the ministry of defence, BBC reported.
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.