Turkey said footage and information from the northwestern Syrian province of Idlib showed that the Syrian regime had used chemical weapons in an attack on Tuesday in what Ankara said was a clear violation of United Nations Security Council resolutions.

“Images and information obtained from Khan Sheikhoun show that the regime has clearly violated the U.N. Security Council’s 2118th and 2209th resolutions by using chemical weapons,” the Turkish foreign ministry said in a statement.

It called on the Organization for the Prohibition of Chemical Weapons (OPCW) to begin an immediate investigation and urged “the sides that influence the Syrian regime” to help put a stop to what it said were heavy ceasefire violations.

The suspected chemical attack killed at least 58 people, including 11 children, a monitor, medics and rescue workers in the rebel-held area said. A Syrian military source strongly denied the army had used any such weapons.