Earlier, three waves of explosions shook Tripoli and its eastern suburbs as NATO kept up its pressure on the Libyan strongman.
Four blasts shook Tripoli at around 2:30 am (0030 GMT) 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 explosions at midday on Sunday in Tajura, a suburb often targeted by NATO since an international coalition began military operations against Libya on March 31.
"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.
Gates, on a visit to Afghanistan, said Kadhafi's aides will inevitably abandon him.
"It's just a question when everybody around Kadhafi decides it's time to throw in the towel and throw him under the bus."
British Foreign Secretary William Hague on Saturday met leaders of rebels fighting to oust Kadhafi 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," he 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 the 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.
Hague in Benghazi on Saturday met the head of the opposition National Transitional Council, Mustafa Abdul Jalil, and toured a medical centre treating war wounded.
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," Jalil said.
Britain said on Sunday 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 an operational update 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.
Neighbouring Tunisia, meanwhile, kept up the grim task of attempting to recover the bodies of more than 200 migrants fleeing Libya's conflict after their boat capsized, a coastguard said.
And Iman al-Obeidi, who grabbed world headlines after accusing Kadhafi's soldiers of torturing and raping her, left Benghazi for the United States, her sister told AFP, after she was deported without explanation by Qatar on Saturday.