An air raid near a bakery and a medical clinic in Syria's rebel-held Idlib region has killed 10 civilians, a war monitor and a rescue service said, as government forces kept up a ground offensive.

The attack on Thursday, believed to have been carried out by Russian warplanes backing a Syrian government offensive, also put a local hospital out of service, they said.

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The reported assault that took place in Ariha, a town in Idlib province, late on Wednesday comes as the rebel-held enclave is under intense fire amid Syrian government advances on the area that had been controlled by the opposition for nearly eight years.

The Britain-based Syrian Observatory for Human Rights (SOHR) said the raid, believed to be carried out by Russian warplanes, killed at least 10 civilians.

The Syrian Civil Defence, a group of rescuers who operate in rebel-held areas, put the death toll at 11, including a child, saying most of them were killed when the Russian warplanes hit a road used by displaced people trying to leave Ariha.

Both the Observatory and the Civil Defence, also known as the White Helmets, said a local hospital and a bakery were struck.

The Ariha hospital, known as al-Shami, was no longer functional, the Observatory said. At least 24 people were wounded, including a doctor, a White Helmets volunteer, three women and two children, the rescuers said.

Russia denies claims

Moscow denied carrying out the attack.

"The Russian aviation did not carry out any combat tasks in this area of Syria," Russia's foreign ministry said in a statement on Thursday, calling foreign media reports a "provocation".

The United Nations Humanitarian Affairs and Emergency Relief Coordinator Mark Lowcock described to the Security Council on Wednesday the dire conditions in rebel-held areas in Syria.

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At least 20,000 people were displaced in the previous two days, he said, adding that 115,000 left their homes in the previous week, bringing the total of those uprooted by the violence since December to 390,000.

"Many families are moving multiple times. They arrive in a place thought to be safe, only for the bombs to follow, so they are forced to move again," he said. "This cycle is all too familiar in northwest Syria."

In the Russian-backed offensive, Syrian troops captured Maaret al-Numan, one of the largest and most strategic rebel-held towns in Idlib province on Wednesday.

The town, which had been in rebel hands since 2012, sits on the highway linking Damascus with Aleppo and is considered critical to President Bashar al-Assad's forces. It was mostly empty after intense bombardment in recent weeks.

The fighting in Idlib has driven hundreds of thousands of civilians from their homes, mainly towards the border with Turkey and other rebel-held areas.