Russia on Tuesday announced the start of what it said was a major offensive targeting extremist militant groups in Syria's north-western province of Idlib and central province of Homs.
"Today we began a large operation to inflict massive fire damage on positions of Islamic State and al-Nusra in the provinces of Idlib and Homs," Russian Defence Minister Sergei Shoigu said in comments carried by the Interfax news agency.
A Russian aircraft carrier off Syria's Mediterranean coast, the Admiral Kuznetsov, has been involved in the offensive, Shoigu told a meeting with Russian President Vladimir Putin.
Idlib province is mostly held by a rebel coalition that includes Jabhat Fatah al-Sham, formerly called the al-Nusra Front, a powerful extremist group with ties to al-Qaeda.
Four towns in the province were hit by strikes, an official with the Nour al-Din al-Zenki rebel group said.
In the northern Syrian city of Aleppo, about 20 kilometres from Idlib province, activists reported that Russian-designed planes were bombing rebel-held held eastern parts of the city.
"At least 14 strikes have hit eastern Aleppo so far, killing four people, among them two children," Ibrahim al-Haj of the Syrian aid group White Helmets told dpa.
An independent monitoring group, the Syrian Observatory for Human Rights, confirmed that several airstrikes hit eastern parts of the city on Tuesday.
Russia has been conducting a bombing campaign against militant groups in Syria for more than a year in support of the Syrian government, a longtime ally. The Syrian government also uses Russian-designed warplanes to conduct bombing runs.
The observatory said it could not immediately determine whether the strikes in Aleppo province had been carried out by Russian or Syrian planes.
Russia said over the past two weeks that it had been observing a unilateral ceasefire in Aleppo to enable civilians and moderate opposition groups to leave eastern parts of the city controlled by the UN-designated terrorist organization Jabhat Fatah al-Sham.
An estimated 250,000 to 300,000 people are under siege in eastern Aleppo, where residents report declining or damaged supplies of necessities including food, water, electricity and medicine.
Syrian government forces stepped up attacks in Aleppo last week, recapturing rebel-held areas on the western outskirts and attempting to move towards eastern areas.
Pro-Syrian regime media have been reporting over the past few days that a large-scale aerial and ground offensive was to be launched on eastern Aleppo, calling on civilians and fighters to leave the city.
The observatory said three hospitals have been hit in airstrikes in the past 24 hours in rebel-held areas on Aleppo's western outskirts.
One hospital was struck overnight in the village of Awijel, while two hospitals were hit in the nearby villages of Kfar Naha and Atarib earlier Monday.
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