COLOMBO, Sri Lanka (AP) _ An explosion demolished a chunk of the government's main building in Jaffna City today, and officials blamed it on a bomb planted by retreating Tamil separatists.

No casualties were immediately reported in the blast in the two-story office of the Government Agent, the state's only liaison in the rebel-held Jaffna Peninsula.

``Fortunately there was no one inside the building,'' said Brigadier Sarath Munasinghe, a military spokesman.

The explosion occurred as troops cleared mines and booby traps from the city center, officials said.

Some 2,300 soldiers and rebels have been killed and more than 6,000 wounded since the government campaign to capture Jaffna _ the deadliest offensive of the 12-year-old civil war _ began Oct. 17.

There has been little fighting since Saturday, which suggests that most of the defenders of the rebel citadel have retreated, the officials said on condition of anonymity.

The Liberation Tigers of Tamil Eelam claimed, however, that fierce close-quarters fighting raged in the town center, slowing the army's progress.

Independent accounts of the battle are unavailable.

Thousands of young men lined up at recruitment centers throughout the country today to volunteer for military service, at the outset of a campaign to enlist 10,000 soldiers for the northern front.

The military's full strength is estimated to be 100,000.

Government troops sealed off central Jaffna on Saturday after capturing the ruins of an 18th-century Dutch fort. Soldiers hoisted the national flag over the rubble of the fort to mark the return of government administration to Jaffna for the first time in five years.

The military said 75 percent of the town was under its control by the weekend.

Although most of Jaffna's 120,000 residents fled weeks ago, troops on Saturday found 400 civilians crowding St. Mary's Church and St. Patrick's College, said Major Tilak Dunuwille, the military spokesman.

Tamil rebels have been fighting for a homeland in northern and eastern Sri Lanka since 1983. They say the nation's minority Tamils suffer extreme discrimination at the hands of the powerful Sinhalese majority.

The rebels have vowed to continue their guerrilla war even if Jaffna city falls. More than 39,000 people have been killed in the war since 1983.