Russian and Syrian fighter jets have reportedly attacked two dozen locations in opposition-held Idlib in northwest Syria.

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”.

Fears for 3m people in rebel-held Idlib ahead of regime offensive Show all 13 1 /13 Fears for 3m people in rebel-held Idlib ahead of regime offensive Fears for 3m people in rebel-held Idlib ahead of regime offensive A Syrian protester waves a flag of the opposition as during a protest against the regime and its ally Russia, in the rebel-held town of Maaret al-Numan in the north of Idlib province AFP/Getty Fears for 3m people in rebel-held Idlib ahead of regime offensive Syrian rebel fighters from the recently-formed "National Liberation Front" stand guard over a trench as rebels prepare defensive positions in anticipation for an upcoming government forces offensive AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive Syrians chant slogans and wave flags of the opposition as they protest against the regime and its ally Russia, in the rebel-held town of Maarrat al-Numan in the north of Idlib province on August 31, 2018. - Its hospitals are battered, residents heavily dependent on aid and escape routes to neighbouring Turkey sealed. If attacked by government forces, Syria's rebel-held Idlib is poised for a humanitarian calamity. (Photo by Zein Al RIFAI / AFP)ZEIN AL RIFAI/AFP/Getty Images ZEIN AL RIFAI AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive Syrian rebel fighters pile-up sandbags AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive Syrians chant slogans and wave flags of the opposition as they protest against the regime and its ally Russia, in Binnish in Syria's rebel-held northern Idlib province on August 31, 2018. (Photo by Muhammad HAJ KADOUR / AFP)MUHAMMAD HAJ KADOUR/AFP/Getty Images MUHAMMAD HAJ KADOUR AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive A Syrian rebel fighter looks through an embrasure in a make-shift bunker AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive A Syrian protester chants slogans as he waves a flag of the opposition as he protest against the regime and its ally Russia, in the rebel-held town of Maaret al-Numan in the north of Idlib province on August 31, 2018. - Its hospitals are battered, residents heavily dependent on aid and escape routes to neighbouring Turkey sealed. If attacked by government forces, Syria's rebel-held Idlib is poised for a humanitarian calamity. (Photo by Zein Al RIFAI / AFP)ZEIN AL RIFAI/AFP/Getty Images ZEIN AL RIFAI AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive Syrian rebel fighters walk through a trench AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive A Syrian protester waves a flag of the opposition AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive A Syrian rebel fighter stands guard over a trench AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive A Syrian rebel fighter from the recently-formed "National Liberation Front" walks through a trench as rebels prepare defensive positions in anticipation for an upcoming government forces offensive, in the countryside of the rebel-held northern Idlib province on August 31, 2018. - In recent days, both the government and its ally Russia have stepped up their rhetoric against the rebel presence in Idlib as they gear up for an assault, which could be the last major battle of the civil war that has torn the country apart since 2011. (Photo by OMAR HAJ KADOUR / AFP)OMAR HAJ KADOUR/AFP/Getty Images OMAR HAJ KADOUR AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive Syrians chant slogans and wave flags of the opposition as they protest against the regime and its ally Russia, in Binnish in Syria's rebel-held northern Idlib province on August 31, 2018. (Photo by Muhammad HAJ KADOUR / AFP)MUHAMMAD HAJ KADOUR/AFP/Getty Images MUHAMMAD HAJ KADOUR AFP/Getty Images Fears for 3m people in rebel-held Idlib ahead of regime offensive Syrian rebel fighters from the recently-formed "National Liberation Front" walk through a a trench as they prepare defensive positions in anticipation for an upcoming government forces offensive, in the countryside of the rebel-held northern Idlib province on August 31, 2018. - In recent days, both the government and its ally Russia have stepped up their rhetoric against the rebel presence in Idlib as they gear up for an assault, which could be the last major battle of the civil war that has torn the country apart since 2011. (Photo by OMAR HAJ KADOUR / AFP)OMAR HAJ KADOUR/AFP/Getty Images OMAR HAJ KADOUR AFP/Getty Images

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.

Russia’s Ministry of Defence has so far declined to comment.