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 Kadhafi'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 Kadhafi 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.
Britain and France said on Saturday they deployed attack helicopters against Kadhafi's forces for the first time as part the NATO campaign to protect civilians in line with UN Resolution 1973.
"We welcome any action that could precipitate the end of (Moamer) Kadhafi's regime," Libyan rebel leader Jalil said.
Britain said on Sunday its Apache helicopters destroyed a multiple missile launcher operated by Kadhafi forces near the eastern oil hub of Brega.
"At sea, HMS Ocean launched her British Army Apaches against a multiple rocket launch system positioned on the Libyan coast near Brega," Major General Nick Pope, spokesman for the Chief of Defence Staff, said in a statement.
"The attack helicopters used Hellfire missiles to destroy their target before returning safely to the ship."
British Tornado strike warplanes separately joined other NATO aircraft in a "major strike on a large surface-to-air missile depot" in Tripoli, Pope added.
In its latest operational update issued on Sunday, NATO said it struck a command and control node, missile storage facility and military installation in Tripoli and rocket launcher, barracks and two checkpoints near Brega.
Moscow, which is calling for a negotiated solution to the conflict, expressed alarm as the NATO campaign entered a new phase.
"We consider that what is going on is either consciously or unconsciously sliding towards a land operation," Foreign Minister Sergei Lavrov said.
"That would be very deplorable," Lavrov, quoted by the RIA Novosti news agency, added of the French and British decision to deploy military helicopters.
It was a concern reiterated on Sunday by Russian Deputy Prime Minister Sergei Ivanov, who expressed doubt that NATO's use of helicopters was an acceptable way to impose a no-fly zone set out under the UN resolution.
"(NATO is) using attack helicopters on land targets, which is in my view the last but one step before the land operation," he told a military forum in Singapore.