The White House said on Thursday that either the Syrian regime or Russia was responsible for a deadly air strike in Syria that hit a school and led to the deaths of 26 civilians.

"We don't know yet that it was the Assad regime or the Russians that carried out the air strike, but we know it was one of the two," White House spokesman Josh Earnest said. "Even if it was the Assad regime that carried it out, the Assad regime is only in a position to carry out those kind of attacks because they are supported by the Russian government."

Russia said earlier on Thursday that Moscow was not responsible for the attack in Idlib.

Air strikes that hit a school in opposition-held Idlib province in northwest Syria killed 22 children, the UN children's agency UNICEF said Wednesday.

In recent months, Idlib, a hub for moderate opposition fighters, has been the target of air bombardment.