SIRTE, Libya – Libyan revolutionary forces claimed to have captured parts of a sprawling convention center that loyalists of Moammar Gadhafi have used as their main base in the ousted leader’s hometown.

The rebels were shelling the city to rout snipers from rooftops in their offensive aimed at crushing this key bastion of the old regime.

The inability to take Sirte, the most important remaining stronghold of Gadhafi supporters, more than six weeks after the capital fell has stalled efforts by Libya’s new leaders to set a timeline for elections and move forward with a transition to democracy.

Gadhafi supporters also hold the enclave of Bani Walid, where revolutionary forces have been stymied by a challenging terrain. But the transitional leadership has said it will declare liberation after Sirte’s capture because that will mean it holds all of the seaports and harbors in the oil-rich Mediterranean coastal country.

British Defense Secretary Liam Fox pledged to keep up NATO airstrikes even after Sirte’s fall, saying the international military action would continue as long as the remnants of the regime pose a risk to the people of Libya.

“We have a message for those who are still fighting for Gadhafi that the game is over, you have been rejected by the people of Libya,” he told reporters Saturday in Tripoli before flying to Misrata.

Revolutionary forces began a major attack on Sirte on Friday after a three-week siege from the outskirts of the coastal city, during which they said they were giving civilians time to flee. On Saturday, fighters fired rockets into the city from the backs of pickup trucks, though visibility was severely limited by a sandstorm.

Libya’s de facto leader, Mustafa Abdul-Jalil, the head of the governing National Transitional Council, said the battle for Sirte has been “ferocious,” with 15 revolutionary fighters killed and 180 wounded on Friday.

“Our fighters today are still dealing with the snipers positioned on the high buildings and we sustained heavy casualties,” he said at a joint news conference in Tripoli.