Two American-built Apache helicopters operating from a British helicopter-carrier ship plying the Mediterranean 20 miles off the Libyan coast attacked targets before dawn near the oil city of Brega. British reporters aboard the ship, Ocean, said that both helicopters returned safely after missions lasting less than two hours.

NATO said in a statement that the helicopters had successfully attacked “military vehicles, military equipment and fielded forces.”

NATO officials have said that they regard the introduction of attack helicopters — the British Apaches and two French helicopter types, the Tigre and the Gazelle — as potential game changers in a conflict that has shown signs of settling into a stalemate. They say the helicopters’ advantage over airstrikes conducted from fast jets flying as high as 20,000 feet is their enhanced ability to carry out precision strikes against Qaddafi forces operating in urban areas, and to pinpoint targets like snipers or small groups of loyalist fighters hiding among civilians or close to schools and hospitals.

But the helicopters also introduce a new level of vulnerability for NATO pilots. Libya still has some scattered air defenses, consisting mainly of portable antiaircraft missiles and truck-mounted systems, which could pose dangers to relatively slow, low-flying helicopters.

The helicopter attacks were begun a week after Prime Minister David Cameron of Britain and President Nicolas Sarkozy of France approved the deployment of attack helicopters from each of their forces, describing the move as intended to increase the military pressure on Colonel Qaddafi. NATO leaders, including President Obama, have called on the Libyan leader to abandon power and leave Libya, demands that he has repeatedly rejected.

Previously, all of 3,700 strike sorties flown during the 11-week air campaign had been flown by fixed-wing attack jets or missile-carrying Predator drones, following up on an initial barrage by cruise missiles.

The British version of the Apache flies at a maximum speed of about 160 miles an hour, compared with the potentially supersonic Tornado, Typhoon and Rafale jets that have led the airstrike campaign so far.

Apaches, long operated in Afghanistan by both the United States and Britain, have an ability to hover high above potential target zones for hours, providing intelligence on enemy movements, as well as the ability to attack multiple targets simultaneously. They can strike with rockets and missiles from distances of five miles or more.

The commander of the British helicopter carrier told reporters aboard the ship that the targets struck early on Saturday included a radar site and an “armed checkpoint” near Brega, a Qaddafi-held port in eastern Libya that has served as a major export point and servicing center for Libya’s oil industry. Control of the port is crucial to the prospects of both the pro-Qaddafi forces and rebel forces in the east, and the town has changed hands several times in recent months.

With the costs of the air campaign mounting, and the stresses growing on air crews that fly multiple missions every week, finding a way of breaking the stalemate has become a priority for NATO, and particularly for Britain and France, which are carrying the brunt of the air campaign.

Mr. Obama has let NATO allies take the lead in the Libyan operations, an unusual role for them in the history of such operations. The United States’ role has been confined primarily to air refueling, airborne command and control and the deployment of missile-carrying drones.

European leaders have shown signs of impatience, concerned that they may have committed themselves to an effort that could last, indecisively, for months or longer. As a result, Mr. Cameron and Mr. Sarkozy have increasingly grown insistent about the need to find ways of bringing a quick end to the conflict by forcing Colonel Qaddafi from power.

Lt. Gen. Charles Bouchard, the Canadian commander who oversees the air campaign from a base in Naples, Italy, issued a statement on Saturday that focused on the new military potential demonstrated by the overnight attacks. “This successful engagement demonstrates the unique capabilities brought to bear by attack helicopters,” he said. “We will continue to use these assets whenever and wherever needed, using the same precision we do in all of our missions.”

Within hours of the Brega attacks, Britain’s Foreign Office announced that Foreign Secretary William Hague, the third-ranking official in the Cameron government, had arrived in the rebel capital of Benghazi in eastern Libya for talks with rebel leaders. He was accompanied by the British minister responsible for foreign aid, Andrew Mitchell. The London statement said that Mr. Hague’ trip was “meant to show support for citizens fighting the rule” of Colonel Qaddafi.