Syrian government forces renewed their assault on the rebellious city of Homs on Tuesday in what activists described as the heaviest shelling in days, as the UN 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” on Tuesday, 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.

UN human rights chief Navi Pillay warned on Monday that the Security Council’s failure to take action has emboldened the Syrian government to launch an all-out assault.

The uprising began last March as mostly peaceful protests against Assad’s authoritarian rule, but has become more militarised in the face of the brutal military crackdown.

Pillay told the General Assembly that more than 5,400 people were killed last year alone, and the number of dead and injured continues to rise daily. She said tens of thousands of people, including children, have been arrested, more than 18,000 reportedly are still arbitrarily detained and thousands more are reported missing. In addition, 25,000 people are estimated to have sought refuge in neighboring countries and more than 70,000 are internally displaced.

“The breadth and patterns of attacks by military and security forces on civilians, and the widespread destruction of homes, hospitals, schools and other civilian infrastructure indicate approval or complicity by authorities at the highest level,” Pillay said.

Also Monday, the Obama administration said it backs Arab League plans to end continuing violence in Syria but noted several obstacles to deploying a proposed international peacekeeping force to the country and withheld full endorsement of the idea.

The administration has said repeatedly it does not see a military solution to the crisis in Syria.