The U.S. on Tuesday condemned Russian and Syrian regime airstrikes in the city of Maarat al-Numan in Idlib province that struck civilian residences and a marketplace.
At least 50 civilians were killed in the air strikes in a de-escalation zone in the province.
"These repeated attacks demonstrate that the Assad regime and its allies continue to believe in and pursue a military solution to this conflict," spokeswoman Morgan Ortagus said in a statement. "These strikes are an act of desperation."
Russian jets struck a marketplace in Maarat Al-Numan on Monday morning, killing 39 civilians, including a White Helmets volunteer, sources from the civil defense agency told Anadolu Agency.
Assad regime warplanes, meanwhile, struck a marketplace in Saraqib in rural Idlib, local sources in the field told Anadolu Agency. The regime airstrike left at least seven civilians dead and dozens of others wounded, they added.
According to an Anadolu Agency correspondent, Russian and regime warplanes also carried out attacks on the town of Bidama and the villages of Talmenes and Al-Kabina, killing four civilians.
At least two more civilians were killed and half a dozen more were injured in strikes that occurred Tuesday.
The Russian Defense Ministry, however, denied its forces targeted a marketplace, saying Russian warplanes had not carried out any missions in the area.
But the U.S. adamantly placed blame on Moscow and Damascus for the attacks.
"Attacks by Russia and the Assad regime also continue to kill and maim humanitarians such as ambulance drivers, health workers, and White Helmets volunteers," said Ortagus. "We call on Russia and the Assad regime to return to a ceasefire in the area, and allow for unhindered access to address the humanitarian disaster created by the airstrikes."
Turkey and Russia agreed last September to turn Idlib into a de-escalation zone in which acts of aggression are expressly prohibited.
The Syrian regime and its allies, however, have consistently broken the terms of the cease-fire, launching frequent attacks inside the area.
On July 12, the Syrian Network for Human Rights (SNHR) said in a report that some 606 civilians had been killed in attacks by regime forces and their allies on the de-escalation zone in Idlib since April 26.
The de-escalation zone is currently inhabited by about 4 million civilians, including hundreds of thousands of people displaced by regime forces from their cities and towns over the course of Syria's more than eight-year conflict.