Libyan rebel forces have attacked the government-held coastal town of Brega, a strategic oil hub in the east of the country.
Medical sources in nearby Ajdabiya said Thursday that one rebel was killed and at least five wounded in the clashes as opposition fighters reported the first columns advancing beyond a front line that had stagnated for weeks.
A Libyan government spokesman said forces loyal to leader Moammar Gadhafi had defeated what he called a coordinated attack by NATO forces and rebels. Moussa Ibrahim told journalists that opposition fighters backed by NATO air and sea forces had attacked Brega. He said the assault violated the alliance's U.N. mandate to protect civilians.
Ibrahim's assessment could not be immediately verified, although Al-Arabiya television also reported that rebels helped by NATO warplanes had attacked Brega from land and sea.
The fighting comes as international diplomats gather in Istanbul to discuss a political solution to the Libyan civil war.
Diplomats expected to attend Friday's talks include U.S. Secretary of State Hillary Clinton, envoys of other NATO members, representatives of Gulf and African nations and Libya's rebel movement. It will be the fourth meeting of the Contact Group on Libya since March, when the uprising began.
Turkey and the African Union each have proposed peace plans to end the war.
Washington and its NATO allies want Mr. Gadhafi to step down immediately to allow Libya to begin a democratic transition. They say their four-month-long campaign of airstrikes on pro-Gadhafi forces will continue as long as those forces keep attacking Libyan civilians.
Mr. Gadhafi said Thursday he will never give in to the rebels or their NATO allies. In a message broadcast by loudspeaker to supporters in the town of Al-Ajaylat, he also called French President Nicolas Sarkozy a "war criminal." France was an early contributor to the NATO mission.
Mr. Gadhafi's prime minister, Al-Baghdadi Al-Mahmoudi, also announced a symbolic end to economic cooperation with Italy, Libya's former colonial ruler and another participant in the NATO airstrikes. Mr. Al-Mahmoudi said Tripoli no longer will work with the Italian government or Italian oil giant ENI, one of the biggest investors in the Libyan oil sector.
ENI already had stopped its oil production in Libya and deliveries of Libyan natural gas to Italy several months ago. ENI's Libyan assets are split between the government-controlled region around Tripoli and the rebel-held regions in the east and west.
Also Thursday, a Russian newspaper quoted Moscow's special envoy to Libya, Mikhail Margelov, as saying Mr. Gadhafi has threatened to blow up Tripoli if the city falls into rebel hands.
Ibrahim, Libya's government spokesman, denied that Mr. Gadhafi had any such plans.
Some information for this report was provided by AP, AFP and Reuters.