Video showed towering flames and thick black smoke billowing from at least two locations in Homs, Syria's third-largest city, which has become the epicentre for the year-long revolt. Residents accused the army of indiscriminate shelling.
"Every day the shelling goes on. The regime is wiping out the city," said an activist who lives in Homs.
Turkey said it had suspended all activities at its embassy as the security situation worsened.
Once a close ally of Mr Assad, Turkey has denounced his efforts to crush the rebellion and has thrown its weight behind his opponents, announcing on Sunday that it would work with Washington to provide "non-lethal" aid to the Syrian opposition.
Kofi Annan, the joint envoy for the United Nations and Arab League, was due to fly to China later in the day as part of an effort to persuade all major powers to put pressure on President Assad to accept the terms of his six-point peace plan.
Mr Annan met Russian leaders on Sunday and won assurances that Moscow was fully behind his initiative, which calls for Mr Assad to back a ceasefire and let in humanitarian aid, but does not demand that he quit – something Western powers are pushing for.
Both Russia and China have previously vetoed UN Security Council resolutions highly critical of Damascus, drawing accusations they were giving President Assad a licence to kill. They argue that the West is too one-sided, but have given full public backing to Mr Annan's mission.
The UN says at least 8000 people have died in the uprising.