Russian and Syrian jets have bombed towns and villages across north-west Syria, devastating civilian areas and forcing fresh waves of refugees to flee to open ground in the biggest aerial blitz on opposition-held areas since the fall of Aleppo more than a year ago.

Monitoring groups said as many as 150 airstrikes beginning on Sunday were recorded in Idlib province by Monday, with dozens more pounding up to 18 towns across the region by nightfall. At least three people were killed on Tuesday morning in the village of Termala, the monitors said.

Residential areas have borne the brunt of the strikes, which severely damaged at least two major hospitals, and levelled dozens of buildings in which panicked locals had taken shelter. Medics claimed nine people were treated for symptoms of chlorine exposure after a bomb was dropped on the town of Saraqeb by a helicopter.

The raids followed the downing of a Russian fighter jet over the province on Saturday and came in the week of a steady increase in violence across Idlib – the last province in Syria to remain outside regime control, where up to 2 million locals and internally displaced people have taken refuge among armed opposition groups, including powerful jihadist forces.

The Russian defence ministry said the fighter pilot, Maj Roman Filipov, killed himself with his own grenade to avoid capture.

The United Nations on Tuesday called for a one-month truce to allow aid to reach hundreds of thousands of civilians in urgent need of assistance.

Refugees and locals say they fear that Idlib has been transformed into a kill box, with the international community paying scant regard to their fate, as regional powers vie for influence in a vital corner of the country.

Under a deal brokered by Russia, Turkey and Iran, Idlib was supposed to be a “de-escalation zone” where fighting was slowed to pave the way for negotiations. But the differing aims of the powers involved in the war in and an insistence by backers of the regime that discussions should be eschewed in favour of all-out military victory has led to the collapse of the accord.

Militias supporting the regime surged into Idlib in December, their first foray into the opposition-controlled province in years, provoking an exodus of more than 200,000 civilians who have fled north to already crowded population centres and further afield to the border with Turkey.

The assault and bombardment have strained Idlib, already home to more than a million refugees from other parts of Syria, who fled there after negotiating surrender deals with the government.

Ahmad al-Dbis, the director of safety and security at the Union of Medical Care and Relief Organisations (UOSSM), which runs hospitals across opposition Syria, said: ”The situation is really bad, because the attacks are targeting all the areas, including Idlib city, Maarat al-Numan, Kafranbel, and a lot of large residential areas, which is leading to a huge number of wounded.

“It’s putting a lot of pressure on hospitals, and at the same time these hospitals are being targeted, so people are being deprived of healthcare. People don’t come to the hospitals because they know they are being targeted.

“There’s a huge displacement to Idlib city and Maarat al-Numan as well as villages and refugee camps. [There are] massacres that the rescue services are unable to cope with because even the ambulances are being hit.”

Hassan Mukhtar, an activist in Idlib province, said: “Maarat al-Numan hospital was bombed by the Russians late at night, it was constant bombardment. The Russians also destroyed the health centre near Saraqeb and the hospital inside Saraqeb.

“They are destroying everything since their pilot went down. We have 300 civilian deaths, mainly women and children, 600,000 internally displaced refugees from the east of Saraqeb and Khan Sheikhun all trying to find a safe haven towards the Turkish border and west of Idlib. Idlib is suppose to be a safe zone; didn’t Russia guarantee it as one? The area is full with civilians.”

Local journalist Souhaib Mokahal said: “The locals were evacuated to nearby farms. Russia soon began shelling and dropping bombs in the west of Idlib. What happened yesterday was very weird: the bombings and rockets had no sounds, they would only emit sounds [long] after they’ve been dropped, which caused a lot of material damage.

“The bombings haven’t stopped. It was so difficult to find corpses and bodies; we are not able to remove anyone. They will try again at dawn.”