US President Donald Trump on Thursday called for the governments in Moscow, Damascus and Tehran to stop the bloodshed that has displaced thousands in Syria's rebel-held province of Idlib.

The heightened regime and Russian bombardment has hit the country's last major opposition bastion since mid-December, as regime forces make advances on the ground despite an August ceasefire and United Nations calls for a de-escalation.

"Russia, Syria, and Iran are killing, or on their way to killing, thousands" of civilians in jihadist-held Idlib, Trump tweeted, adding: "Don't do it!" Nearly 80 civilians have been killed by airstrikes and artillery attacks in the last two weeks, according to the Syrian Observatory for Human Rights, which estimates that more than 40,000 people have been displaced.

Turkey called Tuesday for the attacks to "come to an end immediately," after sending a delegation to Moscow to discuss the flare-up.

Presidential spokesman Ibrahim Kalin said Ankara was pressing for a new ceasefire to replace the August agreement.

Trump praised Turkey's efforts, tweeting that Ankara "is working hard to stop this carnage." In a statement earlier this week, the Syrian army said it had seized 123 square miles (320 square kilometers) from its rivals in recent days.

It has pledged to continue its push until it recaptures all of Idlib, calling on civilians to exit areas under jihadist control.

Idlib is dominated by the country's former Al-Qaeda affiliate, Hayat Tahrir al-Sham.

The head of the group has urged jihadists and allied rebels to head to the front lines and battle "the Russian occupiers" and the regime.

Their "ferocious" campaign "requires us to exert more effort," HTS chief Abu Mohammed al-Jolani said Tuesday in a statement.

Idlib, in northwestern Syria, hosts some three million people, including many displaced by years of violence in other parts of the country. The Damascus regime, which now controls 70 percent of Syria, has repeatedly vowed to take back the area.

Backed by Moscow, Damascus launched a blistering offensive against Idlib in April, killing around 1,000 civilians and displacing more than 400,000 people. Despite a ceasefire announced in August, the bombardment has continued, killing hundreds of civilians and fighters.

The latest spike in violence comes after Russia and China on Friday vetoed a UN Security Council resolution that would have extended for a year cross-border aid deliveries to four million Syrians, many of them in Idlib.

The move raised fears that vital UN-funded assistance could stop entering opposition-held parts of Syria from January unless an alternative agreement is reached. Syria's war has killed over 370,000 people and displaced millions since beginning in 2011 with the brutal repression of anti-government protests.