Syrian government forces seized control of the northern town of Haffah on Wednesday after rebels retreated in the face of a withering offensive that saw the army rely heavily on combat helicopters for the first time.

The fall of Haffah after an eight-day battle marked a setback for the rebel Free Syrian Army, which in recent weeks has been steadily asserting its presence across large swaths of the north and center of the country. Haffah, a mostly Sunni hillside town near the strategic port city of Latakia, had marked the westernmost expansion of the rebels’ reach, giving them a foothold in the regime’s Alawite heartland.

But in recent days, government troops have launched an intensified offensive against rebel strongholds across the country, apparently buoyed by continued indications of Russian support and the failure of international diplomatic efforts to come up with an alternative to a failed U.N. cease-fire plan.

Neighborhoods in the city of Homs came under bombardment for a fifth day, and in the far east, troops were reportedly pushing into the town of Deir al-Zour. The Britain-based Syrian Observatory for Human Rights said 41 people were killed nationwide.

The increased use of attack helicopters, at a time when the rebels are also gaining access to fresh supplies of arms, appears to signal a new escalation in violence 15 months into the popular uprising against President Bashar al-Assad’s rule.

A spokesman for the Free Syrian Army in Turkey said rebels staged what he called a tactical retreat from Haffah to “protect the lives of civilians” after more than eight days under siege. Thousands of civilians, including many wounded, are trying to make their way to safety in Turkey through the mountainous terrain, said Col. Aref Hammoud, who lives in a refugee camp near the southern Turkish town of Antakya.

News reports from Turkey quoted officials as saying that more than 2,000 Syrians have crossed into the country in the past 48 hours to flee the surge in violence.

Syria’s Foreign Ministry called on U.N. observers to visit Haffah, according to a statement quoted by the official Syrian Arab News Agency. It said that government forces had “restored security and calm” to Haffah after clearing it of “armed terrorist groups.”

A U.N. observer mission that tried to visit the town Tuesday was turned back by angry crowds chanting slogans in support of Assad, according to a statement from the monitors and a video of the incident posted online. The footage showed several dozen men swarming around the observers’ vehicles, hitting them with iron rods and jumping on them.

In Paris, French officials escalated the diplomatic rhetoric, calling for a Chapter 7 U.N. resolution that would oblige Syria to comply with the U.N. peace plan, the first step that would be required if the United Nations were to authorize the use of force against Syria.

“We need to move up a gear at the Security Council and place the Annan plan under Chapter 7,” French Foreign Minister Laurent Fabius told reporters. “That is to say, make it compulsory under pain of very heavy sanctions.”

Russia and China have made it clear that they would veto any such resolution.