British army Apache helicopters have attacked Muammar Gaddafi's troops for the first time, destroying a radar station and a military checkpoint and marking a sharp escalation in Nato Libya campaign.

One of the two Apaches came under fire early on Saturday from forces loyal to the Libyan leader near Brega, a strategic oil city close to the eastern frontline, but both returned successfully to HMS Ocean, stationed off the Libyan coast.

French Tiger helicopters struck separately, hitting 15 military vehicles and five command buildings at an undisclosed location.

The new air operation is expected to strongly boost the morale of the rebels, who have been holding ground in various parts of the country but unable to make strong advances towards Tripoli because better-equipped government forces still hold towns along routes to the capital.

The head of the rebel council, Mustafa Abdel Jalil, said: "We welcome any measures that would expedite the departure of Gaddafi and his regime." Jalil later held talks with the British foreign secretary, William Hague, who flew into Benghazi to further pledge Britain's backing for the anti-Gaddafi National Transitional Council.

"Britain remains a strong and true friend of Libya," Hague said. "We could not and did not turn a blind eye when Gaddafi turned his forces against innocent civilians. For as long as Gaddafi continues to abuse his people, we will continue and intensify our efforts to stop him.

"We are here as part of a coordinated and strategic approach to Libya – ensuring that our military, diplomatic and development actions are aligned."

Before the deployment of the Apaches, Nato had relied on jets flying nearly three miles above the ground to target Gaddafi's military arsenal. But the introduction of combat helicopters, which can fly slower at low altitudes, will give the alliance a key advantage in close quarter fighting, allowing it to pinpoint targets in built-up areas where Gaddafi's troops are dug in and reduce the chance of what the military terms collateral damage.

In a statement, Nato said that the craft would allow it to "track and engage pro-Gaddafi forces who deliberately target civilians and attempt to hide in populated areas".

The defence secretary, Liam Fox, said that the use of the attack helicopters was "a logical extension" of Nato's campaign. "This gives us a chance to target new targets in a way we weren't able to do," he said from Singapore, where he was attending a security conference.

"What it does show is our willing to use the range of assets we have to keep the pressure up. We will continue with the methods we have to degrade his [Gaddafi's] command and control, to degrade his supplies."

Nato said that the targets of Saturday's mission included military vehicles, equipment and forces. Major General Nick Pope, the chief of the defence staff's strategic communications officer, said the Apaches, equipped with thermal imaging capabilities, used Hellfire missiles and 30mm cannon during the attack. At the same time, Tornados and Typhoon jets struck another military installation in Brega, and two ammunition bunkers in Waddan, in central Libya. "Our understanding of the detailed disposition of Colonel Gaddafi's forces has been improving in a very satisfactory manner, despite their efforts to conceal themselves," Pope said.

There was no word on casualties. In Tripoli, where jets have been conducting nightly raids on Gaddafi's military infrastructure, the government offered no comment. The use of the low-flying helicopters does raise the possibility of western forces suffering casualties for the first time in the war.

But Lieutenant General Charles Bouchard, commander of Nato's Libyan operation, said the mission was a success and indicated that the helicopters would be used again. "We will continue to use these assets whenever and wherever needed, using the same precision as we do in all of our missions," he said.

Rebels forces are concentrated in three main areas spread across Libya. In the east, where the revolution started in February, the rebels are trying to push towards west from Ajdabiya. Brega is the next town along the highway. Separately, rebels also control Libya's third city, Misrata, 130 miles (210km) from Tripoli, as well as several small towns in the western Nafusa mountains range.

Gaddafi has become increasingly reclusive in Tripoli as the Nato bombing campaign has intensified. While members of his regime continue to defect, and his diplomatic isolation is near complete, he remains defiant, and has refused to entertain suggestions that he leave power or go into exile.

The strikes came after Libyan rebels on Friday won control of four towns in the western Nafusa mountain range, where government forces have besieged and randomly shelled rebel-held areas for months. After weeks of siege, government forces drove about seven tanks and a number of armoured vehicles into Yifran in early May and surrounded nearby Galaa, said Col Jumaa Ibrahim of the region's rebel military council.