Six powerful explosions have been heard in the Libyan capital, Tripoli, as warplanes flew over the city.
A powerful but distant blast was felt in the centre of the city at around 9:00pm (1900 GMT) on Sunday, followed by stronger explosions a few minutes later, an AFP correspondent said, unable to immediately determine the targets.
NATO fighter jets earlier launched intensive air raids on the capital and its eastern suburbs.
A Libyan government official, speaking to the Associated Press news agency on condition of anonymity, said British jets had hit a military barracks in Tripoli, but there were no casualties.
For months, Muammar Gaddafi's forces have been battling opposition fighters who are seeking to end his four-decade rule. Despite mounting international pressure, including NATO air attacks against him, the Libyan leader refuses to step down.
'No deadline'
The Tripoli explosions came as William Hague, the British foreign secretary, said there is no deadline for NATO's Libya operation, and Russia voiced concerns that the use of helicopters showed NATO was sliding towards a land campaign.
Hague, who held talks with Libyan opposition leaders in their stronghold Benghazi on Saturday, ruled out putting ground forces, saying NATO would stick to the terms of a UN Security Council resolution passed in March to protect civilians.
"We will continue in that way, intensifying what we're doing - the Apache helicopters are an example of that - but that's different from mission creep," he said.
"This is not mission creep, changing the nature of the mission, this is intensifying what we are doing in order to make this mission a success."
Hague's trip came just hours after British Apache helicopters attacked forces loyal to Gaddafi. Hague held talks in Benghazi with the head of the opposition Interim National Council, Mustafa al-Jalil. He also toured the city's landmark seafront as well as a medical centre treating war wounded.
"We're encouraging the National Transitional Council to put more flesh on their proposed transition - to lay out in more detail this coming week what would happen on the day that Gaddafi went - who would be running what, how would a new government be formed in Tripoli?" Hague told the BBC.
Opposition forces progress
After a siege by pro-Gaddafi forces, Misurata is now in opposition hands.
Opposition fighters there have now pushed halfway to the town of Zlitan, on the way to Tripoli, after taking control of Zintan.
At one stage, their advance came to within 60km of Sirte, but the government troops held their line and repelled the attack.
Gaddafi's government has been slowly crumbling from within. A significant number of army officers and several cabinet ministers have defected, and most have expressed support for the opposition.
NATO attacks on the Libyan military and government infrastructure have been occurring daily since March 31 in an operation that has just been extended for another 90 days.
PHOTO CAPTION
British Foreign Secretary William Hague (C) tours Revolution Square in the Libyan opposition's stronghold of Benghazi.
Al-Jazeera