A deadly airstrike hit a refugee camp in war-torn Syria on Thursday, activists said.
The pro-opposition group Syrian Observatory for Human Rights said 28 people were killed at the site in Idlib province near the border with Turkey.
It was unclear who launched the airstrike. A State Department spokesman said the U.S. had not determined whether or not Bashar Assad's regime was responsible, but was investigating the reports.
"We're looking into them and trying to get more details on what happened," State Department deputy spokesman Mark Toner said at a briefing.
Toner said the airstrike appears to have hit a refugee camp. White House press secretary Josh Earnest said it is not believed there were any U.S. or coalition aircraft operating in the region.
"There is no justifiable excuse for carrying out an airstrike against innocent civilians who have already once fled their homes to escape violence," Earnest said. "These individuals are in the most desperate situation imaginable, and there is no justification for carrying out military action that's targeting them."
Earnest added that if it turns out Assad's government was responsible for using force against civilians, "it would not at all be the first time."
Video posted online by a group called Syria Civil Defense purported to show the aftermath of the strike, with workers using hoses to douse fires amid burning tents and other wreckage, covering bodies with sheets, and what appeared to be the body of a child.
The strike reportedly happened near the town of Sarmada, about 27 miles west of Aleppo — where an airstrike on a hospital killed more than a dozen doctors and patients last week. The U.S. blamed the Syrian government for the April 27 attack.
British Foreign Secretary Philip Hammond condemned the airstrike on the refugee camp and appeared to blame the Assad regime for continued violence despite international attempts at a cease-fire.
"Reports of the bombing of a refugee camp in Samarda this evening are horrifying," Hammond said in a statement. "The Assad regime's contempt for efforts to restore the cessation of hostilities in Syria is clear for all to see."
U.N. Humanitarian Chief Stephen O'Brien said Thursday that if a refugee camp had been deliberately targeted "it could amount to a war crime."
O'Brien said initial reports indicate that, in addition to the dead, over 80 people were injured in Thursday's attack, among them women and children. "I call for an immediate, impartial and independent investigation into this deadly incident," he added, describing the incident as an "obscene attack."
The United Nations estimates that at least 250,000 Syrians have been killed during the five year conflict. Other estimates are higher. The Syrian Center for Policy Research said in a report released in February that about 470,000 Syrians have been killed through 2015.