Russian warplanes have reportedly bombed the rebel-held Syrian province of Idlib, as government troops mass ahead of what may be a major offensive.
If confirmed, they would be the first such air strikes there in three weeks.
Earlier, US President Donald Trump warned Syria’s Bashar al-Assad against launching a“reckless attack” on Idlib.
But Kremlin spokesman Dmitry Peskov rejected the warning and said the Syrian army was“getting ready” to clear a “cradle of terrorism” there.
Mr Peskov said the al-Qaeda-linked jihadists dominating Idlib were threatening Russian military bases in Syria and blocking a political solution to the civil war.
The UN has warned of a humanitarian catastrophe if an all-out assault takes place.
The Syrian Observatory for Human Rights, a UK-based monitoring group, reported that Russian jets had carried out about 30 raids on about 16 rebel-held areas in western Idlib, the mountains of Latakia province, and the Sahl al-Ghab plain.
The Syria Civil Defence, whose rescue workers are commonly known as the White Helmets, reported that three civilians had been killed in the strikes on Jisr al-Shughour.
The Syrian Observatory said the Russian air strikes were the first for 22 days and came hours after three pro-government fighters were killed by rebel rocket fire in the Jabal Turkmen area of Latakia.
Steps taken by pro-government forces
Syrian army soldiers and allied militiamen have been gearing up for what has been described as a phased offensive on Idlib, the rebels’ last remaining stronghold.
HTS, which is designated by the UN as a terrorist organisation and has an estimated10,000 fighters in Idlib, and rival rebel factions backed by neighbouring Turkey have said they will fight back.
On Monday night, Mr Trump warned Russia and Iran, which has sent military advisers and thousands of militiamen to Syria, that they “would be making a grave humanitarian mistake to take part in this potential human tragedy”.
On Tuesday, Mr Peskov questioned the US president’s approach to solving the problem of HTS and other jihadists operating in Idlib.
“To just make some warnings, not taking into account a very dangerous negative potential of the whole situation in Syria, is probably an incomplete, not all-encompassing approach,” he said.
The Kremlin spokesman said situation in Idlib would be on the top of the agenda at a summit of the presidents of Russia, Iran and Turkey in Iran on Friday.
Agitation by international community
UN special envoy Staffan de Mistura has warned of a “perfect storm” if the government goes ahead with a full-scale offensive.
The jihadists must be defeated but not at the expense of thousands of civilian lives, Mr de Mistura told reporters last week.
He called for further talks on a political solution, or for humanitarian corridors to be set up to allow civilians to be evacuated temporarily to a safer area, most likely one under government control.
The UN says Idlib is home to some 2.9 million people, including a million children. More than half of the civilians have already been displaced at least once from elsewhere in Syria and have nowhere left to go.
“A worst-case scenario in Idlib will overwhelm capacities and has the potential to create a humanitarian emergency at a scale not yet seen through this crisis,” John Ging of the Office for the Co-ordination of Humanitarian Affairs warned.
UN officials say as many as 800,000 people could be displaced and that the already high number of people in need of aid could increase dramatically.