The Syrian government launched an assault to capture rebel-held areas of Aleppo last month with Russian air support and Iranian-backed militias, a week into a ceasefire agreed by Washington and Moscow.

Kerry broke off talks with Russian Foreign Minister Sergei Lavrov last week over the offensive, which has included air strikes on hospitals that Washington and Paris said amounted to war crimes for which Syria and Russia were responsible.

The Syrian and Russian governments blamed their foes for breaking the ceasefire and said they target only militants in the city, the last major urban stronghold of the Western-backed rebels, where more than 250,000 people are trapped under siege.

The resumption of talks, despite the offensive, was a sign of the lack of options facing Western nations over the Syria conflict, where they worry scaled-up arms supplies for the rebels could end up in the hands of jihadist groups.

Russia`s Foreign Ministry said Kerry and Lavrov would meet Lavrov in the Swiss city of Lausanne to consider steps toward settling the conflict. The meeting will include foreign ministers from Turkey, Qatar, Saudi Arabia and Iran. A senior State Department official confirmed Kerry would attend.

On Wednesday, 25 people were killed by heavy air strikes on rebel-held areas of Aleppo, the Civil Defence, a rescue service working in rebel-held areas, said on Twitter, adding that 15 of them were killed at a market place in the Fardous district.

The Syrian army has denied targeting civilians.

A Syrian military source said warplanes had struck several locations to the south and southwest of Aleppo but Syrian and Russian officials could not immediately be reached to comment on the market place attack.

It was the second day of heavy air strikes after a lull of several days which the Syrian army said was designed to allow civilians to leave.

Rebels said the intensity of the air strikes on Tuesday and Wednesday had returned to the level seen at the start of the Russian-backed campaign to capture Aleppo.

Moscow`s intervention just over a year ago tipped the scales of the conflict back toward President Bashar al-Assad.

The Syrian Observatory for Human Rights, a Britain-based organization that reports on the war, said it had documented the deaths of 55 people killed in escalating bombardment of eastern Aleppo in the last 48 hours.

The Observatory also reported that four people were killed in government-held areas by insurgent shelling.

Clashes between pro-government and rebel forces were also reported in a rebel-held area in southern Aleppo.

The war has killed more than 300,000 people, created the world`s worst refugee crisis, allowed for the rise of Islamic State and drawn in regional and major powers.

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