At least 30 people were killed and more than 80 others injured on Thursday in airstrikes on two settlements sheltering refugees in a rebel-held area in northern Syria, the United Nations said, citing initial reports.
UN humanitarian affairs chief Stephen O'Brien said he was "horrified and sickened" by the attacks that targeted camps of internally displaced Syrians in Idlib province near Syria's border with Turkey.
The Syrian Observatory for Human Rights, the monitoring group that first reported on the attacks, said the airstrikes targeted a camp in the town of Sarmada and put the initial death toll at 28 with more than 50 people wounded.
The dead included women and at least seven children, the watchdog said.
The aircraft that carried out the airstrikes were not immediately identified, but some unconfirmed reports said they belonged to regime forces.
"We've seen early claims that this was regime strike, but we just want to be absolutely sure before we level blame at somebody," US State Department Mark Toner said in Washington. "But it's totally in keeping with the types of ... airstrikes that regime has carried out ... against innocent civilians."
"If this obscene attack is found to be a deliberate targeting of a civilian structure, it could amount to a war crime," O'Brien said.
The bombardment came hours after a temporary truce took effect in the northern city of Aleppo, which has been the battleground of deadly fighting between regime forces and rebels in the past two weeks.
Toner said the US stood by its understanding that the truce began at midnight on Wednesday, though the Syrian government said it started 24 hours later. He also said that while the Russians and Syrian government said the cessation would be for only 48 hours, "our intent is to work towards extending that, to make it as open-ended as possible."