At least six powerful explosions were heard from central Tripoli early Saturday evening, a Reuters correspondent in the Libyan capital said.
Aircraft could be heard overhead at the time of the blasts, before sunset. NATO has been bombing targets connected with leader Moammar Gadhafi's government since March.
Minutes after the explosions were heard, Libyan television said in a written news flash: "The city of Tripoli is undergoing attack now by the colonialist crusader aggressors."
NATO forces have been striking targets in and around the capital most nights, usually shortly before or after midnight. Strikes during daylight hours, like Saturday's, are relatively rare.
The military alliance says it is hitting Gadhafi's military to protect civilians after his forces launched a crackdown against an uprising. Leaders of Western countries say they will not stop bombing until Gadhafi steps down.
Earlier, British Apache and French attack helicopters struck targets for the first time in NATO's campaign in Libya, hitting Gadhafi's troops near a key coastal oil city, the alliance said.
The action was a significant step-up in NATO's operations and a major boost to Libyan rebels on the ground, just a day after rebel fighters forced government troops from three western towns and broke the siege of a fourth in yet another erosion of Gadhafi's power since the eruption in mid-February of the uprising to end his 42-year rule.
NATO said the helicopters struck Gadhafi troops attempting to hide in populated areas, military vehicles and equipment. Lt. Gen. Charles Bouchard, commander of the Libya operation, said the engagement "demonstrates the unique capabilities brought to bear by attack helicopters."
The Apaches hit two targets near the coastal city of Brega, according to a statement from the Ministry of Defense in London. It said they took off from HMS Ocean, stationed off the Libyan coast and returned safely after completing their mission in the early morning hours.
British Maj. Gen. Nick Pope said the Apaches targeted a radar installation and a military checkpoint. "Hellfire missiles and 30mm cannon were used to destroy the targets," he said.
Separately from the helicopters, Royal Air Force ground attack aircraft destroyed another military installation near Brega and two ammunition bunkers at the large Waddan depot in central Libya, Pope said.
in simultaneous attacks on different targets, the BBC reported.
"This successful engagement demonstrates the unique capabilities brought to bear by attack helicopters," Bouchard said in a NATO statement released earlier. "We will continue to use these assets whenever and wherever needed, using the same precision as we do in all of our missions."
Attack helicopters provide additional flexibility to track and engage pro-Gadhafi forces who deliberately target civilians and attempt to hide in populated areas, NATO said.
NATO forces are constantly reviewing operations and use of available assets, including attack helicopters, to best maintain the momentum and increase the pressure on pro-Gadhafi forces, NATO said.
The helicopters' use should mean less chance of civilian casualties in operations that previously relied on the use of Tornado and Typhoon aircraft, the BBC said. However, Apaches operate at lower altitudes and could be targeted by Libya's thousands of surface-to-air missiles.
China on diplomatic front
The attacks followed confirmation that China made its first confirmed contact with Libyan rebels, the latest diplomatic setback for Gadhafi. France said it was working with people close to the veteran ruler to persuade him to leave power.
The meeting in Qatar between a Chinese diplomat and the leader of the rebel National Transitional Council follows a spate of defections by high-profile figures this week, including senior oil official and former prime minister Shokri Ghanem.
Libyan rebels and NATO have made Gadhafi's departure a condition for agreeing a cease-fire in the conflict, but he emphatically told visiting South African President Jacob Zuma this week he would not leave Libya.
The NATO-led military alliance extended its mission to protect civilians in Libya for a further 90 days this week, and France said it was stepping up military pressure as well as working with those close to Gadhafi to try to persuade him to quit.
Slideshow 18 photos
Conflict in Libya, Week 15
"He is more and more isolated," French Foreign Minister Alain Juppe told Europe 1 radio. "There have been more defections around him and we have received messages from his close entourage which has understood that he must leave power."
In Beijing, a terse Chinese Foreign Ministry statement said Beijing's ambassador to Qatar, Zhang Zhiliang, had met and "exchanged views on developments in Libya" with Mustafa Abdel Jalil, the chairman of the rebel council that is trying to offer itself as a credible temporary alternative to Gadhafi.
The ministry gave no details of the talks but the meeting itself was an indication that Beijing wants to keep open lines of communication with the rebel forces.
In Tripoli, government spokesman Moussa Ibrahim said he had no immediate comment on China's meeting with the rebels.
Clashes on the ground
Now in its fourth month, the Libyan conflict is deadlocked, with rebels unable to break out of their strongholds and advance toward Tripoli, where Gadhafi appears to be entrenched.
Rebels control the east of Libya around Benghazi, where the rebel council is based, and a mountain range stretching from the town of Zintan, 95 miles south of Tripoli, toward the western border with Tunisia.
In London, Maj. Gen. John Lorimer said British warplanes destroyed two tanks and two armored personnel carriers on Thursday in Yafran, 60 miles southwest of Tripoli, where he said Gadhafi's forces were continuing to attack rebels.
A rebel spokesman in the Western Mountains town of Nalut, Kalefa Ali, said rebels had been fighting forces loyal to Gadhafi in Trumeet to the south of Yafran and in Bir Ayyad.
The rebels took over Bir Ayyad, a road junction which controls access to Yafran from the north, and replaced the green Libyan flags there with the red, black and green rebel flag, a Reuters cameraman said.
"Bir Ayyad is under rebel control," a rebel called Mohammed told Reuters by telephone. He said no rebel fighters were wounded in the fighting.
In Misrata, rebel leader Abdelsalam reported fighting in Dafniyah to the west of the city. "It started at 10:00 a.m. (4:00 a.m. ET) and Gadhafi's forces have been shelling Dafniyah since then, using mortars and heavier rockets," he said.
He said revolutionaries in Zlitan had been supplied with weapons and telecommunications equipment from Misrata and Benghazi, and had been waging attacks at night, "but not on the scale that threatens Gadhafi's forces' iron grip on Zlitan."
Zlitan is the next town to the west of Misrata, and one of only three between there and Tripoli. A rebel spokesman there, Mabrouk, said security in the town was tight.
"The brigades have a heavy presence," he said. "They are going around with a special scanner that tracks satellite phones. They know revolutionaries use those. Today they also blocked the main road to Tripoli."
