* Ghanem defection would be serious setback for Gadhafi

* Tripoli says has no indication he has left his post

* Deaths in clashes on Tunisian border, in Misrata

TRIPOLI/TUNIS - The chairman of Libya's National Oil Corporation has defected from Muammar Gadhafi's administration and fled to neighbouring Tunisia, a Tunisian security source said on Tuesday.

Libyan rebels also said they had information that Shokri Ghanem, 68, had defected, a move that if confirmed would deal a major blow to Gadhafi's efforts to shore up his 41-year rule.

"He is in a hotel with a group of other Libyan officials," the Tunisian source told Reuters, saying Ghanem had been staying in the south of the country. Another Tunisian security source said he was on his way to the capital Tunis.

A government official in Tripoli said there was no sign Ghanem had defected.

Three months into the unrest, rebels hold Benghazi and the oil-producing east of Libya, helped by a NATO bombing campaign sanctioned at the United Nations to protect civilians.

But the military victory rebels once sought seems a distant prospect and many pin their hopes on a collapse of central power in Tripoli driven by disaffection and defections.

Ghanem, an internationally respected technocrat, is credited with liberalising Libya's economy and energy sector.

Rebel finance and oil minister Ali Tarhouni told Reuters on a visit to Doha that he understood Ghanem had left his post, and said he hoped to represent Libya at an OPEC meeting in June. Libya is estimated to have lost two thirds of its oil output since the unrest began three months ago.

Rebels and Arab media reported on a previous occasion that the U.S.-educated Ghanem had stepped down but he appeared later and said he was in his office and working as usual.

The most prominent Libyan defector so far is Moussa Koussa, the former foreign minister, who fled to Britain earlier this year.

BORDER FIGHTING

Libyan state television said Libyan forces had hit a NATO warship that was shelling areas of the rebel-held city of Misrata, but NATO denied the report, saying it was "totally fabricated".

A doctor in the western city said at least seven people were killed during fighting between rebels and government forces earlier in the day. Most of the dead were believed to be rebels.

British aircraft and navy ships attacked a training base for bodyguards for Gadhafi's inner circle and intelligence buildings in the capital, the British Defence Ministry said on Tuesday.

In central Tripoli, NATO airstrikes hit two buildings including one a Libyan spokesman said contained files detailing corruption cases against government officials who have defected.

Officials summoned reporters after the attack in the early hours to visit the two damaged buildings, which they said housed security forces and Libya's anti-corruption agency. One building was in flames.

Thousands have been killed in the Libyan conflict, the bloodiest revolt of what has been called the "Arab Spring" of protests that has removed leaders in Tunis and Cairo.

Libyan rebels briefly pulled back from the Dehiba-Wazin border post on the frontier with southern Tunisia on Tuesday. Five were killed and several wounded by Libyan government shellfire, they said.

At least four Russian-made Grad rockets fired from Libya landed inside Tunisia during the clashes. The crossing is on a supply route for rebels in Libya's Western Mountains and is an exit point for injured fighters.

Tunisia said it would refer Tripoli to the U.N. Security Council if Libya fired across the border again.

MOSCOW TALKS

Russia hosted a representative of Gadhafi's government in Moscow on Tuesday and called on Tripoli to stop using force against civilians, comply fully with UN Security Council resolutions and withdraw armed groups from cities.

"The answer we heard cannot be called negative," Foreign Minister Sergei Lavrov told reporters. He suggested Gadhafi's government was making such steps conditional on NATO and rebels calling a halt to the use of force.

Libya was ready to look at peace proposals based on those suggested by the African Union and to comply with Security Council resolutions, he said.

"The only things that our interlocutors from Tripoli noted today was the necessity of the insurgents accepting analogous steps and that NATO also stop bombing," Lavrov said, adding that it remained to agree terms and a timeframe for a truce.

Both the rebels and Western nations have rejected past ceasefire proposals, saying the only deal they would accept was one under which Gadhafi relinquished power. The proposal set out by the Russian foreign minister made no mention of that.

The talks indicate Russia's desire to act as peacemaker and preserve its influence in Libya, where it has billions of dollars of arms, energy and infrastructure contracts.