Russia's deputy prime minister has expressed concerns that a land campaign in Libya may be "one step" away for the NATO-led operation to remove Libyan strongman Moamar Gaddafi.

Moscow expressed alarm at the NATO campaign entering a new phrase as Britain and France said on Saturday they deployed attack helicopters against Mr Gaddafi's forces for the first time.

Russia has repeatedly called for a negotiated solution to the conflict.

British foreign secretary William 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.

But Russia's deputy prime minister Sergei Ivanov questioned the tactic, telling a military forum in Singapore that NATO's use of attack helicopters is "the last but one step before the land operation."

Russia's foreign minister Sergei Lavrov says it appears that NATO is headed for a land operation.

"We consider that what is going on is either consciously or unconsciously sliding towards a land operation," he said.

Britain said on Sunday Apache helicopters destroyed a multiple missile launcher operated by pro-Gaddafi 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," he said.

Mr Hague admitted on Sunday that the NATO operation was "intensifying" and that there was no deadline for the mission.

"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.

Five powerful explosions rocked Tripoli late on Sunday as warplanes overflew the Libyan capital. A powerful but distant blast was felt at around 9:00pm (local time), followed by stronger explosions a few minutes later.

US defence secretary Robert Gates said it was only a matter of time before Mr Gaddafi's aides "throw him under the bus".

"I don't think anyone knows how long. But I think you see signs the regime is getting shakier by the day," he said.

"It's just a question when everybody around Gaddafi decides it's time to throw in the towel."

- AFP