The air strikes come as Israel shoots down an Iranian jet flying over the Golan Heights from Syria and US forces evacuate a base near the Iraq border.
A militant offensive in a region of northwestern Syria, where a safe zone deal had brought months of calm, prompted heavy regime and Russian air strikes Tuesday, including on a hospital, a monitor said.
Militant factions led by a former Al Qaeda affiliate, which are not included in the de-escalation deal, launched a fierce assault on a string of regime-held villages along the border between Idlib and neighbouring Hama province.
"An hour later, the regime launched air strikes on the operation's supply lines. Raids are now ongoing across southern Idlib province and northern parts of Hama province," said Observatory head Rami Abdel Rahman, adding that Russian warplanes later joined in.
Syrian army raids hit a maternity hospital in the village of Al-Tah and a medical outpost in the town of Khan Shaykhun, as well as near two other hospitals in Idlib province, Rahman said.
Israel downs Iranian drone
Israel shot down a pilotless plane over the Golan Heights on Tuesday and said it was probably an Iranian-built aircraft on a reconnaissance mission for the Lebanese Hezbollah militant group along Israel's frontier with Syria.
The incident came hours before Prime Minister Benjamin Netanyahu was due to address the UN General Assembly and cite Israeli concerns about an entrenchment in Syria of Iranian and Hezbollah forces that have been helping Damascus beat back rebels.
The drone was launched from an air base near Damascus and was downed with a Patriot interceptor missile over the Golan demilitarised zone that has separated Israeli and Syrian forces since the ceasefire deal that followed their 1973 war, the military said.
"As we understand, it was on a reconnaissance mission along the border and the Golan Heights on behalf of Hezbollah," a military spokesman, Lieutenant-Colonel Jonathan Conricus, told reporters.
"Our intelligence suggests it was ... Iranian-made."
Asked whether the drone might have been armed, Conricus said that information was not yet available.
US troops evacuate base
US troops and allied Arab fighters have evacuated a garrison set up in June in the Syrian desert near the border with Iraq and relocated to their main Tanf base, rebel sources said.
They said the move follows a deal between Washington and Moscow to abandon the Zakf site, located around 60-70 km (40-50 miles) northeast of Tanf.
Zakf was established to stop the Syrian regime army and allied Iranian-backed militias advancing from territory north of Tanf towards the Iraqi border after they managed to cut off and encircle rebels backed by Washington.
"We have left the garrison after our allies reached an agreement with Russia to pull back to Tanf. We have moved all the equipment and destroyed some of the fortifications so they are no longer usable," said a source who belongs to Maghawir al-Thawra.
Maghawir al-Thawra is a Pentagon-backed rebel group that maintained the base and patrols around Tanf.
Syrian regime leader Assad has gained momentum in the six-year civil war but rebel groups still control large, populous areas in both the northwest and southwest of the country as well as other pockets elsewhere.