White House spokesman Josh Earnest said Thursday that only Moscow and the regime of Syrian President Bashar al-Assad had the capabilities to carry out a strike like the one that killed 22 children on Wednesday.

"We know it was one of the two," he 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."

Earlier, French Foreign Minister Jean-Marc Ayrault made similar comments at a news conference: "Who is responsible? In any case it is not the opposition because you need planes to launch bombs."

At least 26 civilians, most of them children, were killed in Wednesday's strike on a village in Syria's northwestern rebel-held Idlib province.

Russia: 'It wasn't us'

Moscow said it had nothing to do with the school attack and demanded a swift investigation.

According to monitoring group, the Syrian Observatory for Human Rights, "warplanes - either Russia or Syrian - carried out six strikes" in the rebel-held area. The UN children's agency UNICEF said the school complex was "repeatedly attacked."

A Russian Defense Ministry spokesman quoted by Russian news agencies said Russian planes did not enter airspace near the school at the time of the strike. He also said drone footage of damage at the school showing the building with its roof intact was not consistent with an airstrike. 

War crime?

United Nations Secretary-General Ban Ki-moon said he was appalled by the attack and called for an immediate and impartial probe.

"If deliberate, this attack may amount to a war crime," Ban said in a statement. 

"If such horrific acts persist despite global outrage, it is largely because their authors…do not fear justice," he added. "They must be proved wrong."

UN envoy for global education, former British Prime Minister Gordon Brown, called on the UN Security Council to ask the International Criminal Court prosecutor to open an investigation "into what I believe is a war crime." 

In 2014, Russia and China used their Security Council veto power to block a resolution to allow the ICC to look into war crimes in Syria.

Syria's conflict began in March 2011 as a peaceful uprising against the Assad government. It has since escalated into a full-scale war with Syrian regime forces backed by Russia, Iran and Shiite militias fighting against mostly Sunni rebel groups, including some backed by the US, Turkey and Gulf states.

As many as 300,000 civilians have been killed and millions displaced in the nearly six-year conflict.

nm/sms (Reuters, AFP, AP, dpa)