BEIRUT (AP) — Syrian government forces renewed their assault on the rebellious city of Homs today in what activists described as the heaviest shelling in days, as the U.N. human rights chief raised fears of civil war.

Troops loyal to President Bashar Assad have been shelling Homs for more than a week to retake parts of the city captured by rebel forces. Hundreds are believed to have been killed since last Saturday, and the humanitarian conditions in the city have been worsening.

Homs was under “ brutal shelling” today, the Local Coordination Committees activist group said, citing its network of witnesses on the ground.

Another activist group, the Britain-based Syrian Observatory for Human Rights, said it was the heaviest shelling in days.

With diplomatic efforts bogged down, the conflict in Syria is taking on the dimensions of a civil war, with army defectors clashing almost daily with soldiers.