At least four hospitals have been targeted by air raids in Syria's northwestern province of Idlib, killing three people and wounding several, according to a war monitor and rescue teams.

Rami Abdel Rahman, head of the UK-based Syrian Observatory for Human Rights (SOHR), said Syrian government and Russian warplanes conducted the air attack on Tuesday in the countryside of Hama and Idlib.

The attacks came after opposition factions led by a former al-Qaeda affiliate launched an assault on a number of government-held villages along the border between Idlib and Hama.

The raids hit a maternity hospital in the village of al-Tah and a medical outpost in the town of Khan Sheikhoun, as well as near two other hospitals in Idlib province, Abdel Rahman told AFP.

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"The raids on the maternity hospital in al-Tah killed an elderly woman who was an employee of the hospital, in the first civilian death in air strikes in Idlib province since May," he said.

According to the medical charity UOSSM, which has provided equipment to the maternity hospital, the woman killed was a cleaner in the facility and other medical staff were wounded.

In a statement distributed to journalists, the charity said two sets of attacks hit the maternity hospital, setting fuel-storage tanks on fire and leaving its incubator room "totally destroyed".

The Syrian government-affiliated Central Military Media outlet acknowledged the opposition offensive in Idlib and said that forces responded with intensive aerial bombardment and shelling, killing a number of fighters.

UOSSM condemned the attacks and said that "systematic attacks on hospitals are back despite de-escalation deals".

Idlib is mainly controlled by Hay'et Tahrir al-Sham, an alliance of anti-government groups formed in January and linked to al-Qaeda.

It is led by Jabhat Fateh al-Sham, formerly known as al-Nusra Front.

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The SOHR said the air raids were in retaliation for the killing of 19 soldiers when Hay'et Tahrir al-Sham launched an attack on government forces in the northern Hama countryside.

The fighting comes just days after Iran, Russia and Turkey announced they would jointly police the de-escalation zone in Idlib and parts of adjacent Hama and Latakia provinces.

Russia has already deployed military police to the other three de-escalation zones - Eastern Ghouta near Damascus, parts of the south and some areas of the central province of Homs.

The agreement excludes the Islamic State of Iraq and the Levant (ISIL) group and Hay'et Tahrir al-Sham.