(CNN) Airstrikes were reported Saturday in the Syrian province of Idlib, the last remaining rebel stronghold in the war-torn country, according to volunteer rescue group the White Helmets and the Syrian Observatory for Human Rights.
Both groups reported that Russian aircraft targeted the area.
Four people were killed and five wounded in the strikes, the White Helmets said on Twitter, also posting images it said were from the attack.
The UK-based observatory reported "intensive" airstrikes Saturday in Idlib as well.
The strikes occurred a day after Russia, Turkey and Iran failed to agree on a ceasefire in Idlib at a trilateral summit in the Iranian capital.
On Monday, President Donald Trump warned Syria and its allies Russia and Iran against attacking Idlib province in the country's northwest. Hours after the tweet, Russian planes struck parts of western Idlib, according to an activist media group and the White Helmets.
US officials have grown worried that an assault on Idlib could involve the use of chemical weapons if the rebels are able to slow regime advances.
The Syrian regime has regained control of much of the country over the past few years, but Idlib has remained a bastion of rebel support.
The regime, supported by Russia and Iran, is preparing to retake the province, an area crammed with as many as 70,000 rebel fighters and nearly 3 million civilians, double its pre-war population.
Experts say if that happens, the battle for Idlib could be the last act of a seven-year civil war that has reduced much of Syria to ruins and killed an estimated 400,000 people.