* French and UK helicopters hit targets in Brega
* Powerful explosions heard in Tripoli
* British foreign secretary arrives in rebel stronghold
(Adds explosions in Tripoli, Hague quote, Biden on Gaddafi)
By Peter Graff
TRIPOLI, June 4 (Reuters) - British and French attack helicopters struck inside Libya for the first time overnight on Saturday, hitting targets in the oil port of Brega as NATO forces stepped up their air war against Muammar Gaddafi.
Aircraft of the NATO-led alliance also hit targets in Tripoli, where at least six powerful explosions were heard. A Reuters correspondent in the Libyan capital said aircraft could be heard overhead at the time of the blasts, before sunset.
It was not immediately clear which targets were hit.
"As long as Gaddafi continues to abuse his people, we will continue and intensify our efforts to stop him from doing so," British Foreign Secretary William Hague said at a news conference in the rebel stronghold of Benghazi in eastern Libya.
Speaking in Benghazi shortly before Hague's arrival, the head of the rebel council Mustafa Abdel Jalil welcomed NATO's deployment of helicopters.
"We welcome any measures that would expedite the departure of Gaddafi and his regime," he told reporters in Benghazi, where Hague later arrived for talks with council members.
Hague visited the square next to Benghazi's court house where people greeted him by waving the victory sign and shouting: "Libya free!" and "Gaddafi go away!"
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A 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 Gaddafi to try to persuade him to quit.
"This was the first operational mission flown by British Army Apaches at sea," British Defence Secretary Liam Fox said.
"The additional capabilities now being employed by NATO further reinforce the UK's enduring commitment and NATO's determination to ... ensure that the people of Libya are free to determine their own future."
Military analysts say attack helicopters will allow more precise strikes against pro-Gaddafi forces hiding in built-up areas than the high-flying jets used so far, while reducing the risk of civilian casualties.
But given the vulnerability of helicopters to ground fire, their deployment also increases the risk of Western forces suffering their first casualties of the campaign.
Critics of the war have warned of "mission creep" but NATO has said the use of helicopters would not presage the deployment of ground troops, which Western nations have ruled out.
Now in its fourth month, the Libyan conflict is deadlocked, with rebels unable to break out of their strongholds and advance towards Tripoli, where Gaddafi appears to be entrenched.
Rebels control the east of Libya around Benghazi and the Western Mountains stretching from the town of Zintan, 150 km (95 miles) south of Tripoli, towards the border with Tunisia.
In a speech at NATO's Libya command base in the Italian city of Naples, U.S. Vice President Joe Biden praised U.S. troops for taking part in military operations that he said had stopped a humanitarian disaster in the North African country.
He reiterated the official U.S. government stance that Gaddafi must step down.
Rebel fighters repelled an attack by Gaddafi's forces against one of their checkpoints on the eastern edges of the rebel-held city of Misrata on Saturday, a Reuters journalist there said. One rebel was killed and another was wounded in the clashes, medical workers said.
Gaddafi's forces also shelled Nalut and Zintan, rebel spokesmen said by phone from the rebel-held Western Mountains towns. At least 10 people were wounded in Nalut.
NATO's helicopter attacks struck military targets around the eastern town of Brega, location of an oil export terminal.
Rebel forces swept west through Brega early in the uprising before retreating from near Gaddafi's home town of Sirte in late March. Gaddafi's forces have since dug in around the oil town.
"The Apaches were tasked with precision strikes against a regime radar installation and a military checkpoint, both located around Brega," said Major-General Nick Pope, the Chief of the Defence Staff's Strategic Communications Officer.
"In the same area, Royal Air Force ground attack aircraft destroyed another military installation, whilst a separate RAF mission successfully attacked two ammunition bunkers at the large Waddan depot in central Libya."
In a fresh diplomatic setback for Gaddafi on Friday, China said it had made its first confirmed contact with Libyan rebels this week, following a spate of defections by high-profile figures including senior oil official and former prime minister Shokri Ghanem.
The rebels and NATO have made Gaddafi's departure a condition of any ceasefire, but he emphatically told visiting South African President Jacob Zuma this week he would not leave Libya.
The United Nations has said government-held parts of Libya are running out of food and the capital Tripoli this week saw the first big protest in months against Gaddafi's 41-year rule.
Gaddafi says the rebels are armed criminals and al Qaeda militants, and has called the NATO intervention an act of colonial aggression designed to grab Libya's plentiful oil. (Additional reporting by Zohra Bensemra in Misrata, Abdelaziz Boumzar in Bir Ayyad, Libya, Gavin Jones in Italy, John Irish in Paris, Christina Fincher in London, Sherine El Madany in Benghazi, David Brunnstrom in Brussels and Joseph Nasr in Rabat; writing by Lin Noueihed and Joseph Nasr; editing by Andrew Roche)