BENGHAZI (AFP) - Nato pounded Tripoli on Sunday hours after Britain's top diplomat met rebel chiefs in Libya and Russia voiced concerns the alliance's military operation is sliding towards a land campaign. US Defence Secretary Robert Gates said it was only a matter of time before aides to Libyan leader Moamer Gaddafi abandon him. Warplanes launched intensive air raids on Tripoli and its eastern suburbs as NATO kept up its pressure on the embattled Libyan strongman. British Foreign Secretary William Hague on Saturday met leaders of rebels fighting to oust Gaddafi after NATO deployed attack helicopters for the first time. Hague admitted on Sunday that the NATO operation was "intensifying" and that there was no deadline, but denied any "mission creep" for the aerial bombing campaign launched nearly three months ago. "We're not going to set a deadline. You're asking about Christmas and who knows, it could be days or weeks or months, (but) it is worth doing," Hague told BBC television. Hague defended the use of attack helicopters and ruled out putting forces on the ground in Libya, saying NATO would stick to the terms of a UN Security Council resolution passed in March to protect civilians. "This is not mission creep, changing the nature of the mission, this is intensifying what we are doing in order to make this mission a success," he added. Gates, on a visit to Afghanistan on Sunday, said Gaddafi's aides will inevitably abandon him. "It's only a matter of time (before he falls)," Gates said. "I don't think anyone knows how long. But I think you see signs the regime is getting shakier by the day. "It's just a question when everybody around Gaddafi decides it's time to throw in the towel and throw him under the bus." Hague on Saturday held talks in Benghazi with the head of the opposition National Transitional Council, Mustafa Abdul Jalil, and toured a medical centre treating war wounded. Three waves of explosions have rocked Tripoli since he departed the rebel stronghold. Four blasts shook Tripoli at around 2:30 am (0030 GMT) on Sunday after two powerful but distant explosions were felt in the city centre at around 6:30 pm on Saturday, followed by others within minutes. Witnesses reported four more at midday on Sunday in Tajura, a suburb often targeted by NATO since an international coalition began military operations against Libya on March 31.