At least 28 people were killed and more than 50 others injured on Thursday in airstrikes on a refugee camp in a rebel-held area in northern Syria, a monitoring group said.
The Syrian Observatory for Human Rights said the airstrikes targeted a camp in the town of Sarmada in Idlib province near Syria's border with Turkey.
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.
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."