BEIRUT - Syrian government forces recaptured four northwestern villages on Tuesday as they pounded the area with air strikes in a counter-attack on insurgents threatening strongholds of President Bashar al-Assad, a monitor said.

Government warplanes by Tuesday afternoon had launched more than 100 air strikes since the previous night on parts of the Sahl al-Ghab plain seized by rebels in an advance this month, the Britain-based Syrian Observatory for Human Rights said. This month’s insurgent advance into the plain had brought rebels, including the al Qaeda-linked Nusra Front, to the eastern edge of mountains that form the traditional heartland of Assad’s Alawite people, forcing an army retreat to new defensive lines. The four-year-long war has gathered pace recently, intensifying on major frontlines including near Damascus, where a government air strike on a marketplace this week killed 100 people, and in the southern city of Deraa, where the government is battling a new rebel attempt to seize the entire city.

All are areas of vital importance to Assad, who with help from his regional allies Iran and the Lebanese group Hezbollah is seeking to shore up control over western areas of the country after losing much of the rest to groups including Islamic State.

The rebel advance into Sahl al-Ghab not only threatened the Alawite mountains but opened a route south to the city of Hama, one of the western population centres seen as a priority for Assad. A source in the Syrian military, which by Assad’s own admission faces a manpower problem, said the army was advancing in the Sahl al-Ghab area. Pro-rebel activists on Twitter reported fierce battles between an alliance of insurgent groups and government forces in the area.

The Observatory also reported advances by Hezbollah and the army in the rebel-held town of Zabadani near the Lebanese border, where a ceasefire collapsed at the weekend after talks between the warring sides failed to agree a longer cessation of hostilities. Government forces dropped 25 barrel bombs on the town, which is of crucial importance to Assad and Hezbollah because of its location at the Lebanese border and proximity to Damascus, the Observatory said.

The army also launched a third day of air strikes on the town of Douma to the northeast of Damascus, the target of Sunday’s marketplace air raid, the Observatory said. Fighting between rebel and government forces was also reported in the nearby area of Harasta. Sunday’s air strike in Douma drew condemnation from the United States, which says Assad has lost the legitimacy to rule.

The U.N. envoy for Syria, Staffan de Mistura, also condemned the air strike - criticism that appeared to draw an attack from the Syrian government which said he was “straying from neutrality”. The violence underscores the huge challenges facing a new diplomatic effort to advance potential solutions to the conflict that has killed an estimated quarter of a million people and driven more than 11 million from their homes.

At a meeting of their two foreign ministers on Monday, Russia and Iran, whose support has been crucial to Assad’s survival, said his future must be decided by Syrians. Moscow also said it opposed any pre-negotiated exit of the Syrian president as part of a peace deal. The Observatory estimates Assad controls a quarter of Syria, including cities where the bulk of the population live.