At least 500 people, including 89 civilians, have been killed since the Russian-backed offensive on Aleppo province began earlier this month, a monitoring group has said.

The UK-based Syrian Observatory for Human Rights reported on Wednesday that at least 500 people had been killed since the Syrian government, backed by Russian air strikes, launched a major offensive from the north of Aleppo on February 1.

The Observatory said that among those killed were "89 civilians, including 23 children, 143 pro-government fighters, 274 rebels and foreign fighters".

Tens of thousands of Syrians have fled their homes in Aleppo's suburbs and are waiting on the Turkish border.

The Syrian government holds the west of Aleppo city while the rebels hold the east, but the situation is largely reversed in the countryside.

The UN warned on Tuesday that up to 300,000 people living in the city of Aleppo could be cut off from humanitarian aid unless access could be negotiated.

"If government advances around the city continue," it said, "local councils in the city estimate that some 100,000 to 150,000 civilians may flee," the UN Office for Coordination of Humanitarian Affairs said.

Doctors Without Borders (MSF) said on Wednesday that at least 23,000 Syrians had arrived in camps around the town of Azaz, east of Aleppo.

At one border crossing with Turkey known as Bab al-Salameh, MSF said there were around 79,000 Syrians staying in camps.

Russian air strikes

Russia launched its military operation in Syria in September 2015, and it says the campaign is against armed groups such as the Islamic State of Iraq and the Levant (ISIL) and al-Nusra Front.

Turkish Prime Minister Ahmet Davutoglu accused Russia on Wednesday of carrying out a deliberate policy of ethnic cleansing around the city of Aleppo.

On Wednesday, Russia denied such accusations and said its air strikes have not targeted civilians in Syria.

Maria Zakharova, the Russian foreign ministry’s spokeswoman, said that "Moscow has still not received convincing evidence of civilian deaths as a result of Russian air strikes in Syria."

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Zakharova also said that Russia's operation in Syria "is directed exclusively at fighting terrorist threats".

Syrian peace talks were suspended in Geneva this month until February 25, after UN envoy Staffan de Mistura said more work was needed to make progress.

The UN Security Council is expected to discuss the humanitarian situation in Syria in a closed-door meeting later on Wednesday.

The Syrian opposition has said that it will not attend the scheduled talks unless the government ends its air strikes and lifts the sieges on cities and towns.

The peace talks are meant to develop a "road map" to end the conflict of almost five years that has resulted in more than 250,000 Syrians being killed.

The conflict has also displaced millions more and sent hundreds of thousands fleeing as refugees to Europe.