Troops entered the northern town of Kilinochchi from two sides, senior military officials told The Associated Press on condition of anonymity in line with government rules.
The level of fighting inside the town remained unclear. The military had predicted Thursday that soldiers would seize the town within two days.
The fall of Kilinochchi would be devastating to the separatist Tigers, who have had used the town as their political and military headquarters for the past 10 years and have created structures for an independent state, such as a police, courts, and tax offices.
Recent government military offensives have forced the rebels out of much of their territory in the north of the Indian Ocean island nation. President Mahinda Rajapaksa has promised to crush the rebel group and end the nation's 25-year-old civil war this year.
A 6-year-old Norway-brokered truce was officially called off last year with both sides openly violating the agreement.
Senior Sri Lankan officials have said repeatedly over the past two months that Kilinochchi would fall soon, but troops became bogged down by heavy rains and fierce rebel resistance.
The rebels could not immediately be reached for comment. But Tamil Tiger political leader Balasingham Nadesan told The Associated Press on Tuesday that they began as a guerrilla group and would be able to keep fighting even if they lost much of the territory they controlled in the north.
Military spokesman Brig. Udaya Nanayakkara said that 15 insurgents were killed in daylong fighting near the rebels' northeastern stronghold of Mullaitivu on Thursday. Thirteen soldiers were wounded in the battle, he said.
The rebels have fought since 1983 to create an independent homeland for the minority Tamils, who have suffered decades of marginalization by successive governments controlled by the Sinhalese majority. The conflict has killed more than 70,000 people.
