Syrian government forces have renewed their assault on the rebellious city of Homs, activists said, as the UN human rights chief raised fears of civil war.
The Local Co-ordination Committees activist group said Homs came under "brutal shelling" early on Tuesday.
Syrian forces have been shelling the city for more than a week. The latest onslaught began last Saturday to retake parts of the city captured by rebel forces.
With diplomatic efforts bogged down, the conflict 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.
Ms Pillay told the UN 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 neighbouring 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," she said.