World powers pressed Russia to stop bombing around Aleppo in support of a Syrian government offensive to recapture the city. Secretary of State John Kerry is pushing for a ceasefire and more aid access to Aleppo, where rebel-held areas are being cut off and the United Nations has warned a new humanitarian disaster could be on the way.

Syrian Information Minister Omran al-Zoubi said the government expected a tough but relatively short battle to return the city to state control. Russian U.N. Ambassador Vitaly Churkin said Russian air strikes were being undertaken in a “transparent manner” and some Security Council members had “crossed the line” by politically exploiting humanitarian issues.

The UK-based Syrian Observatory for Human Rights reported on Wednesday that at least 500 people have 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  among those killed are “89 civilians, including 23 children, 143 pro-government fighters, 274 rebels and foreign fighters”.

The Syrian government holds the west of Aleppo city, Syria’s largest, while the rebels hold the east, but the situation is largely reversed in the countryside. The conflict has also displaced millions more and sent hundreds of thousands fleeing as refugees to Europe.