US special envoy says the air raids were carried out by the Syrian government and its ally Russia.
There have been 200 air strikes on opposition-held territory in northern Syria in the last three days, mainly targeting civilians, the US has said.
Special envoy James Jeffrey told reporters the attacks in Idlib province had been carried out by the Syrian government and its ally, Russia.
In one of the latest raids, 11 civilians were killed in reported Russian air strikes near a bakery and a clinic in the town of Ariha.
Russia denied its forces were involved.
Fighting in Idlib, the last stronghold of the opposition to President Bashar al-Assad, has intensified in recent weeks as the Syrian army pressed on with an offensive.
On Wednesday, soldiers captured the strategically important town of Maarat al-Numan, which straddles a highway linking the capital Damascus with Aleppo.
