(Reuters) - Russia believes Libyan leader Muammar Gaddafi should quit and could help broker his departure, a senior Russian official said on Friday in an important boost to NATO powers bent on ending his 41-year rule.

It was a striking change in tone from Kremlin criticism of Western air strikes in Libya officially intended to protect civilians in a civil war but effectively taking the side of rebels seeking Gaddafi's removal and democratic change.

NATO said it was preparing to deploy attack helicopters over the Arab North African state for the first time to add to the pressure on Gaddafi's forces on the ground.

But his security forces demonstrated once again that they are far from a spent force, launching rocket attacks overnight on the rebel-held town of Zintan and fighting insurgents on the outskirts of the city of Misrata.

The Russian mediation offer was announced on the sidelines of the Group of Eight summit in Deauville, France, where Russian President Dmitry Medvedev was among the heads of state in attendance.

"Colonel Gaddafi has deprived himself of legitimacy with his actions. We should help him leave," Russian Deputy Foreign Minister Sergei Ryabkov said in Deauville.

He said Russia would use its dialogue with the Libyan authorities to "help Mr Gaddafi take the right decision."

Earlier, U.S. President Barack Obama said he had agreed with French counterpart Nicolas Sarkozy that the only acceptable outcome was for Gaddafi to go. "We are joined in resolve to finish the job," he said.

ATTACK HELICOPTERS

A NATO-led coalition led by France and Britain has been bombing Libya since March, under a U.N. mandate to protect civilians caught up in a battle with rebel forces intent on ending Gaddafi's 41-year rule.

But the rebels' advance toward Tripoli has been checked hundreds of km (miles) short of their goal, creating a quandary for Western powers who want a quick outcome in Libya but also to avoid getting embroiled in another Middle Eastern conflict by putting troops on the ground.

Britain and France have tried to help break the deadlock by agreeing to deploy attack helicopters over Libya. They will be able to give close ground support to rebel forces, though they are also at greater risk of being shot down.

British Prime Minister David Cameron said in Deauville that NATO's war in Libya was entering a new phase and that the deployment of British helicopters would turn up the pressure on the Libyan leader.

Gaddafi has denied attacking civilians, saying that his forces were forced to act against armed criminal gangs and al Qaeda militants. He says the NATO intervention is an act of colonial aggression aimed at grabbing Libya's bountiful oil.

GADDAFI'S STATE OF MIND

There was skepticism that Gaddafi would agree to step aside as part of any negotiated settlement, even with Russia now joining calls for his departure.

"Knowing his state of mind, I don't think he is going to step down... The positions are still very far apart (between the rebels and Tripoli)," Arab League Secretary-General Amr Moussa said on the sidelines of the G8 summit.

Previous attempts at mediation -- by the African Union, Turkey and the United Nations -- have also foundered on Gaddafi's refusal to leave and the rebels' refusal to accept anything less.

"I don't see a negotiated settlement having much traction at this stage. Gaddafi still doesn't seem the type to leave the country and the opposition has no incentive to cut a deal," said Henry Smith, a Libya analyst with Control Risks.

Gaddafi's prime minister, Al-Baghdadi Ali Al-Mahmoudi, said on Wednesday Libya was ready for a ceasefire, but Gaddafi's departure was not up for discussion.

"The leader Muammar Gaddafi is the leader of the Libyan people; he decides what the Libyan people think. He is in the hearts of the Libyan people," the prime minister said.

MISRATA BATTLE

Rebel-held Misrata, Libya's third-biggest city and scene of some of the fiercest battles in the three-month-old conflict, was hit by a second day of heavy fighting on its western outskirts.

A Reuters reporter said he could see white puffs of smoke and dust from where mortars fired by pro-Gaddafi forces were landing.

The insurgents responded by firing back with rockets and heavy machine guns, shouting "Allahu Akbar!" (God is Greatest) after each volley.

Doctors at Misrata's hospital said three rebels were killed and 16 wounded in the fighting on Friday.

"We are being attacked from all sides with rockets, RPGs (rocket-propelled grenades) and mortars," said Faraj al-Mistiri, 36, an insurgent. "They are trying their hardest to get back into Misrata," he said.

The World Health Organization said the fighting in Misrata had been killing an estimated 12 people a day, though casualties had declined after fighting eased in the past week.

The WHO did not give a total figure, but its daily estimate would mean a total of about 925 killed over the 77 days of intense fighting in Misrata.

ROCKET STRIKES

Gaddafi's forces intensified their attacks too on the town of Zintan, part of a chain of mountain-top settlements in the Western Mountains, near Libya's border with Tunisia, where rebels have been holding off assaults for months.

A foreign doctor in Zintan, about 150 km southwest of Tripoli, said the town came under intense rocket fire overnight from pro-Gaddafi forces positioned to the east.

"There must have been about a hundred (strikes). I wasn't counting, but there were four or five rockets every half an hour or 15 minutes," Anja Wolz of Doctors Without Borders said by telephone.

Wolz said it was a "miracle" no one had been seriously hurt. She said Zintan's hospital was relocating to Jadu, another rebel-held town about 18 km (11 miles) west of Zintan.

"Zintan is emptying, people are leaving," she said.

(Reporting by Joseph Logan in Tripoli, Mohammed Abbas in Misrata, Souhail Karam in Rabat, Matt Robinson in Tataouine, Tunisia, David Brunnstrom in Brussels, Steve Holland, Keith Weir, Alexei Anishchuk and Nicolas Vinocur in Deauville, France and Barbara Lewis in Geneva; Writing by Christian Lowe; Editing by Mark Heinrich)

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