An Islamist militant offensive in a region of northwestern Syria where a safe zone deal had brought months of relative calm prompted heavy government and Russian air strikes on Tuesday, a monitor said.
Idlib province and some adjacent areas form one of four so-called de-escalation zones agreed in May by rebel backer Turkey and government allies Russia and Iran.
Front lines across the province had been relatively quiet since then but intense fighting erupted on Tuesday, the Syrian Observatory for Human Rights said.
Islamist 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 government-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.
He said Russian warplanes later joined in the strikes, which wounded dozens of civilians.
"The attack and air strikes are the most intense in the area since the de-escalation zone was announced in May," Abdel Rahman told AFP.
At least 17 government troops and militia were killed, the Observatory said. Twelve Islamist militants and two medics who had been working with them also died.
State news agency SANA also reported heavy clashes and said that government warplanes had hit the opposing forces' supply lines.
The fighting comes just days after Iran, Russia and Turkey announced they would jointly police the safe zone in Idlib and parts of adjacent Hama and Latakia provinces.
Russia has already deployed military police to the other three safe zones -- Eastern Ghouta near Damascus, parts of the south and some areas of the central province of Homs.
The de-escalation agreement excludes the militants of the Islamic State (IS) group and Hayat Tahrir al-Sham, an alliance dominated by Al-Qaeda's former Syrian affiliate.