Russia has denied carrying out air strikes on a school in Syria's rebel-held Idlib province that killed 36 people, including 22 children.
"The Russian Federation has nothing to do with this terrible tragedy, with this attack," foreign ministry spokeswoman Maria Zakharova said on Thursday, adding Moscow demanded an immediate investigation.
Zakharova said claims that Russian and Syrian warplanes had conducted the deadly air strikes in Idlib on Wednesday were "a lie".
The raids hit a school and the surrounding area, killing 22 children and six teachers, the United Nations children's agency said.
The incident prompted outrage from UNICEF director Anthony Lake.
"This is a tragedy. It is an outrage and - if deliberate - it is a war crime," he said, adding the school complex had been hit repeatedly.
The UK-based Syrian Observatory for Human Rights monitor group said "warplanes - either Russian or Syrian - carried out six strikes" in the village of Hass, including on a school complex.
The White House also said on Thursday that either the Syrian government or Russia was responsible for a deadly air strike in Syria that hit the school.
"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."
UN Secretary-General Ban Ki-moon called on Thursday for an immediate investigation of the attack.
Ban said in a statement that the attack, carried out against rebel-held territory, may amount to a war crime if found to be deliberate.
"If such horrific acts persist despite global outrage, it is largely because their authors, whether in corridors of power or in insurgent redoubts, do not fear justice. They must be proved wrong," he said.
READ MORE: Aleppo - Russia under fire at Human Rights Council
President Vladimir Putin said on Thursday that Russia had no other option but to clear out what he called "a nest of terrorists" from Aleppo, despite the fact that civilians were also present in the city.
Putin said civilian casualties in conflicts should be mourned everywhere, not just in Aleppo, pointing to what he said were civilians killed around Mosul in Iraq.
"Bells should toll for all innocent victims. Not just in Aleppo," said Putin.
Russia's defence ministry said on Thursday that Syrian and Russian warplanes had not bombed Aleppo in the past nine days.
Meanwhile, the Syrian state media reported that at least six children were killed and 15 others wounded in the government-held west of Aleppo city by two rebel rocket attacks on Thursday.
OPINION: Russia overplays its hand in Syria
One of the attacks struck a school in Shahba neighbourhood, leaving three children dead and 14 others injured, SANA said. The second rocket attack hit a house in the Hamdaniyeh neighbourhood, killing three brothers and injuring a fourth.
Rebel groups have not responded to the reports.
Syrian forces backed by Russia, Iran, and the Lebanese group Hezbollah have waged an aerial and ground assault since late September to recapture eastern Aleppo, killing hundreds of civilians and destroying infrastructure, including hospitals.
Also on Thursday, at least eight people were killed in government shelling on the rebel-held district of Douma in Damascus province, the Syrian Observatory said.
Douma is regularly targeted by government fire, and in recent months regime forces have waged an offensive in the area, which has also been under siege since 2013.
Source: Al Jazeera News And Agencies