GENEVA (Reuters) - A tank shell hit the United Nations office in western Aleppo on Sunday, damaging the top floors of a building that is well known to be the U.N. base in the contested Syrian city, a U.N. statement said on Monday.

“It is appalling that the building that houses the U.N. offices was directly targeted,” the top U.N. official in Syria, Ali Al-Za’tari, said in a statement.

“We strongly condemn the increased violence in all of Aleppo, east and west, which has resulted in the death and injury of scores of civilians, including children.”

The U.N. statement did not mention any people being hurt by the explosion, nor did it specify how it was known to be a tank shell, rather than any other type of munition, or which side was responsible for firing the shell.

Rebel groups have launched an assault on western Aleppo in the past few days to try to lift a siege on the eastern half of the city, where an estimated 275,000 people, and 8,000 rebels, are surrounded by forces loyal to President Bashar al-Assad.

Over 40 people have been killed and many more wounded by rockets indiscriminately launched by non-state armed groups on civilian areas in western Aleppo, the U.N. statement said.