Continued shelling of the rebel-held western outpost of Misrata illustrated the scale of the problem facing rebel forces and NATO. Rebels said Gaddafi forces were trying to advance into the long-besieged city under cover of rocket and mortar shells.

Hospital officials said two people were killed and several wounded in Monday's fighting in Misrata. Later in the day heavy explosions outside the city were heard, lasting about an hour.

A rebel spokesman said forces loyal to Gaddafi also shelled the rebel-held town of Zintan and massed troops close to another town in the mountainous region bordering Tunisia, intensifying operations on the war's western front.

Confirming the proposed use of helicopter gunships, French Foreign Minister Alain Juppe told reporters in Brussels the move was in line with a United Nations resolution to protect Libyan civilians and NATO's military operations.

"What we want is to better tailor our ability to strike on the ground with ways that allow more accurate hits," he said. "That is the goal in deploying helicopters."  NATO bombing has damaged Gaddafi's armour but not enough to break a deadlock between rebels and government forces. While helicopters could make it easier to strike urban or embedded targets, they would also be more vulnerable to ground fire from Gaddafi's troops.

The French daily Le Figaro reported that 12 helicopters, which could launch more precise attacks on pro-Gaddafi forces and targets than fixed-wing aircraft, were shipped out to Libya on the French warship Tonnerre on May 17.

"It is not just French helicopters ... it's coordinated action by the coalition," the diplomatic source said, in response to the newspaper report. "It is at NATO level."

The source said the move could not be considered as part of a strategy to use ground troops in the conflict, now in its fourth month.

A NATO official said he did not know whether the helicopters would come under NATO command. "NATO is aware that the French government has decided to send another ship to join operations in the Mediterranean under national command. Coordination of this ship's activities with NATO operations could take place in the future if and so when required," he said.

A U.N. Security Council resolution allows NATO to strike Gaddafi forces in defence of civilians, but it explicitly excludes any military occupation. Critics such as Russia accuse NATO of overstepping their mandate in prosecuting a systematic campaign to force an end to Gaddafi's 41-year rule.

British Foreign Secretary William Hague said he agreed it was necessary to intensify pressure on Gaddafi's forces, but declined to say if Britain planned join a helicopter deployment.

According to Le Figaro's source, French special forces, who have been operating in Libya to help identify targets for NATO planes since the start of air strikes, could now be reinforced and deployed to guide helicopter attacks.

The use of helicopters, while it could allow NATO forces to launch more accurate attacks, would pose additional risks for NATO. Helicopters would fly lower and be more exposed than aircraft flying well above depleted air defences.The downing of helicopters could draw ground forces into rescue efforts.

"Twelve helicopters is not a lot," Ken Freeman, associate fellow of the Royal United Services Institute for Defence and Security Studies (RUSI). "They tend to be quite vulnerable, so they are probably going to be used very carefully .. You could probably say it is a sign that people are running out of ideas."

Juppe said: "This is not a change of strategy. The strategy remains the same: protecting the population by weakening Gaddafi's military power and military might is not just about armoured vehicles and planes, but also command centres and supply structures."

Intensifying diplomatic activity ahead of a G8 meeting of world powers in France this week, the most senior U.S. diplomat to visit during the uprising arrived in the eastern city of Benghazi for talks with leaders of the rebellion.

Jeffrey Feltman, assistant secretary of state for the Near East, met members of the National Transitional Council formed to administer the eastern regions under rebel control, on the heels of EU foreign policy chief Catherine Ashton's visit on Sunday.

"We are here for the long term and what we can offer is support to Libyan institutions and the economy. We will be here to support you all the way," Ashton said.

Gaddafi describes his opponents as religious extremists, criminals and foreign-backed mercenaries. He says he has no intention of stepping down after the manner of Tunisian and Egyptian autocratic leaders overthrown in an "Arab Spring" of democratic protest that swept the Middle East.

Gaddafi should quit in order to allow a peaceful transition of power, Turkish Foreign Minister Ahmet Davutoglu said on Monday, standing alongside Mustafa Abdul Jaleel, head of the rebels' national transitional council, after talks in Ankara.

A major Muslim partner in NATO, Turkey earlier this month proposed a timetable for a ceasefire that could allow a political transition to unfold. Davutoglu said the rebel council  "a legal and credible representative of the Libyan people".

Russian Foreign Minister Sergei Lavrov said he would meet a Libyan "opposition" delegation on Monday to try to promote a ceasefire and negotiations.

"It is important at this stage to agree a makeup of participants in future talks -- which I hope will be soon but are inevitable in any case -- that would represent the interests of all the political forces, all the tribes in Libya."

The rebels have refused proposals for a ceasefire and talks from the Gaddafi administration, arguing that he has broken previous unilateral ceasefires. They insist Gaddafi, his allies and his family must renounce power as part of any settlement.

As rebel hopes of a military victory have faded, Gaddafi opponents in Libya and Western governments have sought the collapse of the Libya administration from within, encouraging defections of senior officials.

Tunisia said on Monday Libya's top oil official was in Tunisia and believed to be no longer working for Gaddafi. There has been doubt about Shokri Ghanem's fate since rebels said last week he had defected -- a charge Tripoli has denied, saying he was merely on an official trip to Tunisia, Europe and Egypt.

Separately, Tunisia's official news agency TAP said a number of Libyans, including Foreign Minister Abdelati Obeidi and Libya's ambassador to Liberia, had crossed into Tunisia at Ras Jadir. It was not immediately clear why they were travelling.