BEIRUT (Reuters) - Air strikes by Syrian or Russian warplanes on Wednesday killed at least 26 people, most of them schoolchildren, in a village in Syria's rebel-held Idlib province, rescue workers and a monitoring group said.

The raids hit a residential area and a school in Haas village, the Syrian Civil Defence rescue workers' network said on its Facebook account.

Syria's civil war pits President Bashar al-Assad, backed by Russia, Iran and Shi'ite Muslim militias from Lebanon, Iraq and Afghanistan against an array of mostly Sunni Muslim rebel groups, including some backed by Turkey, Gulf monarchies and the United States.

A report on Syrian state TV quoted a military source as saying a number of militants had been killed when their positions were targeted in Haas, but made no mention of a school.

Russia's Ambassador to the United Nations Vitaly Churkin said: "It's horrible, I hope we were not involved. It's the easiest thing for me to say no, but I'm a responsible person, so I need to see what my Ministry of Defence is going to say."

Anthony Lake, the head of the U.N. children's agency UNICEF, said of the attack: "If deliberate, it is a war crime."

"This latest atrocity may be the deadliest attack on a school since the war began more than five years ago," Lake said in a statement.

Idlib, which is in northwest Syria near Aleppo, contains the largest populated area controlled by rebels - both nationalist groups under the banner of the Free Syrian Army and Islamist fighters including the former al Qaeda affiliate Jabhat Fateh al-Sham.

The Civil Defence network, which operates in rebel-held areas in the country, said 20 of the dead in Wednesday's attacks were children.

Photos taken at the scene showed buildings with walls reduced to rubble, including what appeared to be the school with upturned desks and chairs covered in dust.

The Syrian Observatory for Human Rights, a British-based war monitor, said the warplanes had struck several locations in Haas including an elementary and middle school, killing at least one teacher as well as children, though it gave a lower toll of 15 children killed.

Western countries and international human rights groups have regularly highlighted the high number of civilian deaths reported after Syrian and Russian air strikes.

(Reporting by Ellen Francis and John Davison, additional reporting by Michelle Nichols at the United Nations; editing by Andrew Heavens, G Crosse)