President Donald Trump is demanding Syria, Russia and Iran stop "bombing the hell out of Idlib province in Syria."
Trump said in a late Sunday tweet that the bombings are "indiscriminately killing many innocent civilians."
"The world is watching this butchery. What is the purpose? What will it get you? STOP!" he wrote.
Trump, in comments to reporters at the White House Sunday, also noted his call last year for Syria and its allies to avoid an all-out offensive in Idlib due to humanitarian concerns.
Russian President Vladimir Putin expressed similar sentiment in late April, but his government has also warned that the presence of militants in Idlib was undermining attempts to end Syria's eight-year conflict.
Idlib is in northwestern Syria along the Turkish border. It is the last major part of Syria still controlled by rebels trying to topple President Bashar al-Assad.
Rebels and their families who surrendered other parts of Syria under the threat of a bloody offensive have crowded into Idlib looking for safety.
The rebels, who still hold the province, have so far refused to give up. Syrian and Russian forces and, as Trump says, "to a lesser extent" Iran, have sharply increased their bombardment and rocket attacks on Idlib.
The surge in violence comes months after Turkey, Russia and Iran agreed on a cease-fire plan under which Turkey would try to curb militant behavior in Idlib. Turkey is concerned that a major offensive there could send people fleeing across the border into Turkey, which is already hosting 3.6 million Syrian refugees.
The Syrian Observatory for Human Rights said last week the latest violence against Idlib has killed about 950 people.
Elsewhere in Syria, the Observatory says a car bomb in the pro-Turkish rebel-held city of Azaz killed at least 14 people Sunday and wounded more than 20.
Witnesses told the monitor the bomb went off as people were leaving a mosque after evening prayers and meal breaking the daily Ramadan fast.
No one has claimed responsibility for the blast, which also burned or blew out the windows of more than a dozen nearby stores.
The area has been the target of several other recent terrorist bombings.
Azaz is the main city in the part of Aleppo controlled by pro-Turkish rebels, who drove out Islamic State while keeping Kurdish forces out of the area as well.