According to local media, the operation began just before lunchtime on Tuesday and involved Russian Sukhoi Su-24Ms and Su-34s jets, which were supported by Russian ships in the eastern Mediterranean.
The warplanes bombarded countryside around Jisr al-Shughour on the western edge of the rebel enclave of Idlib, after no airstrikes there for three weeks, killing 13 civilians but no fighters, according to the Syrian Observatory for Human Rights monitoring group.
For Syrian president Bashar al-Assad, the defeat of rebels in the northwestern province would mean breaking the last major stronghold of active military opposition to his rule after seven years of conflict. A number of nations, including the US with President Donald Trump being vocal, have warned against the offensive.
Troops loyal to President Assad have massed around the area for weeks, and it seems likely a ground assault will follow.
Half of Idlib’s three million people have already fled from their homes to other parts of Syria, according to the United Nations, and any offensive threatens further displacement.
The attack came just hours after Mr Trump warned Russia, Syria and Iran against attacking Idlib.
Writing on Twitter, the US president said hundreds of thousands could be killed: “The Russians and Iranians would be making a grave humanitarian mistake to take part in this potential human tragedy.”
The White House followed that up with a statement saying that Washington was “closely monitoring” the situation and that any offensive by Syria would be “reckless”. The statement added that if Syria used chemical weapons as it had in the past then the “US and its allies would respond swiftly”.
A Syrian government minister said the siege of Idlib would probably be resolved by force. “Military action is more likely than reconciliation,” Ali Haidar told Russia’s Arabic-language Sputnik news agency. Damascus uses the term “reconciliation” for the negotiated rebel surrenders that have taken place in some areas.
“Idlib is different from other regions because of the large numbers of fighters,” Mr Haidar said. “However we cannot say there is no gateway to reconciliation.”
Any offensive could spark a wider confrontation with Turkey, a supporter of the rebels, whose army has set up observation posts along the Idlib front lines to deter fighting.
Turkey’s Hurriyet daily newspaper reported that Turkish armed forces were reinforcing the Idlib border with M60 tanks.
Idlib’s dominant rebel faction is Tahrir al-Sham, a jihadi alliance spearheaded by al-Qaeda’s former official affiliate in Syria, Jabhat al-Nusra, though other insurgent groups are also present.
On Tuesday, Steffan de Mistura, UN special envoy to Syria, warned against “dramatic” moves. He urged Russia and Syria to continue to negotiate with other major players in the region, Turkey and Assad-ally Iran, as part of upcoming talks.
“We still believe the political process should not be hostage to anything, otherwise there will always be an excuse used by anyone to postpone everything else,” he said. Leaders from the three nations are due to meet in a three way summit in Tehran on Friday.
Russia has rejected both Mr Trump’s and Mr de Mistura’s warnings, and has said the west has deliberately closed its eyes to the nature of the anti-Assad forces who remained in Idlib.
Speaking at a daily press conference, Kremlin spokesman Dmitry Peskov described Idlib as “a terrorist’s nest” that threatened Russian troops deployed in the province.
The situation undermined the peace process and threatened to destabilise Syria, he claimed.
“Just to speak out with some warnings – without taking into account the very dangerous, negative potential for the whole situation in Syria – is probably not a full, comprehensive approach,” he said.
Last week Mr de Mistura said Jabhat al-Nusra and al-Qaeda, both of which the international body designates as terrorists, had an estimated 10,000 fighters in Idlib.
Russian networks carried Mr Peskov’s carefully crafted comments, but they remained silent on the growing reports of the Russian airstrikes well into the evening of Tuesday. State media news agencies carried a brief summary of reports of the airstrikes.