Moscow on Thursday denied any involvement in bloody air strikes on a Syrian school as its relations with the West took another hit and the EU slapped more sanctions on its ally Damascus.

UN Secretary-General Ban Ki-moon demanded an immediate probe into Wednesday's attack on the school in rebel-held Idlib province that he said "may amount to a war crime".

The tensions mounted a day after the United States and Britain said they expected an assault in the next few weeks to drive the Islamic State jihadist group out of Raqa, its de facto capital in Syria.

Syria's conflict broke out in March 2011 with peaceful protests against President Bashar al-Assad's government but has evolved into a complex war involving regional and international powers.

One complication has been the involvement of Turkey, whose President Recep Tayyip Erdogan announced his country's military operation supporting Syrian rebels will also target Raqa.

Russia, whose military intervened in Syria in September 2015, denied having any role in air strikes on the school that the UN children's agency UNICEF said killed 22 students and six teachers.

"The Russian Federation has nothing to do with this terrible tragedy, with this attack," foreign ministry spokeswoman Maria Zakharova said, adding that Moscow demanded an immediate investigation.

Zakharova said claims Russian and Syrian warplanes had conducted the strikes were "a lie".

According to the Syrian Observatory for Human Rights monitor, "warplanes -- either Russian or Syrian -- had carried out six strikes" in the Idlib provincial village of Hass, including on the school complex.

- Children in crossfire -

Russia's defence ministry also denied any involvement.

"On Wednesday, October 26, not one Russian warplane entered that area," spokesman Igor Konashenkov said.

On a nearby front, the ministry said Syrian and Russian warplanes had not bombed the northern city of Aleppo in the past nine days.

A ceasefire meant to allow evacuations of the besieged rebel-held east of Aleppo ended at the weekend, with Moscow ruling out an extension of the unilateral measure for the time being.

Idlib province is controlled by the Army of Conquest, an alliance of rebel groups and jihadists including the Fateh al-Sham Front, which changed its name from Al-Nusra Front after breaking off ties with Al-Qaeda earlier this year.

Syrian and Russian warplanes regularly bomb Idlib, but air strikes have intensified in recent weeks, according to the Observatory.

Children were reportedly caught in the crossfire again Thursday, with state media saying at least six were killed and 15 wounded in rebel rocket attacks on the government-held west of Aleppo city.

The rocket fire hit two west Aleppo neighbourhoods, with one of the attacks striking a school, said the official SANA news agency.

Outside Damascus, meanwhile, a child was among eight people killed Thursday in government shelling on the rebel-held town of Douma, the Observatory said.

- Fresh EU sanctions -

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.

At a makeshift hospital in the town, an AFP photographer saw medics using a defibrillator on one man, his face speckled with blood.

On a stretcher nearby, a wounded man lay with his artificial leg detached and lying on top of him, smeared with his blood.

More than 300,000 people have been killed in Syria since the conflict began.

With nothing seemingly able to stop the bloodshed, the European Union on Thursday added 10 top Syrian officials to its sanctions blacklist for the role in the "violent repression" of civilians.

"The persons... include high-ranking military officials and senior figures linked to the regime," it said.

EU leaders agreed last week to increase sanctions against Assad's regime, citing devastating attacks on Aleppo, Syria's second city and pre-war commercial hub.

Suggestions they might also sanction Russia, which has flown many of the missions against rebel-held east Aleppo, were dropped after sharp differences emerged.

The names of the 10 officials targeted Thursday are to be released at a later date.

The European Commission on Thursday said separately that attacks on schools in Syria were "totally unacceptable", adding "those responsible should be brought to justice".

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