PARIS - France and other members of the NATO-led coalition plan to deploy attack helicopters in Libya, a French diplomatic source said on Monday, a move aimed at ramping up pressure against Muammar Gadhafi's forces.

Twelve helicopters were shipped out to Libya on the French helicopter carrier Tonnerre on May 17, French daily Le Figaro reported earlier, to help break a military stalemate three months into an uprising against Gadhafi's government.

NATO bombing has damaged Gadhafi's armor but not enough to break the deadlock. While helicopters could make it easier to hit urban or embedded targets, they would also be more vulnerable to ground fire by Gadhafi's troops.

French armed forces spokesman Thierry Burkhard declined to confirm the report, but said the ship had left for the Mediterranean on May 17.

"Using helicopters from the Tonnerre is a way of getting closer to the ground," Le Figaro said, citing a source close to the matter.

French planes were the first to bomb Gadhafi's forces in March after the United Nations voted to allow intervention to protect civilians. The airstrikes, now led by NATO, were launched as Gadhafi's troops advanced on Benghazi after the Libyan leader vowed "no mercy, no pity".

Forces loyal to Gadhafi shelled neighborhoods in the west and south of the Libyan city of Misrata on Monday, a rebel spokesman said, adding that explosions were also heard in the port area.

Rebels said they have pushed Gadhafi's forces 25 km from the center of the city after weeks of street fighting and bombardment. Rebel spokesman Abdelsalam said Gadhafi's forces were trying to advance from the west, under the cover of Grad rockets and mortar shells, toward Misrata, scene of the some of the fiercest fighting since the uprising began in mid-February.