Syrian troops on Monday launched a major assault to capture rebel-held areas of the central city of Homs, the Syrian Observatory for Human Rights said.
The offensive comes after at least 264 people were killed across the strife-torn country on Sunday, among them 115 regime soldiers, 104 rebels and 45 civilians, the Observatory said.
"This is the worst fighting in months and there are dozens of dead and wounded among the assailants," the Observatory said without giving further details.
Regular troops backed by pro-regime militiamen attacked the centre of Homs where rebels are holed up, including the Old City and neighbourhoods of Jouret al-Shiah, Khaldiyeh and Qarabees, it said.
Activists refer to Homs as the "capital of the revolution" as the uprising against the regime of President Bashar al-Assad was the most intense in the city of 800,000 residents before regime forces regained control of around 80 percent of it.
Sectarian tensions also run high in the city. Some 65 percent of its population are Sunnis, 25 percent Alawites and around eight percent Christians.
In the northern city of Raqa on the Euphrates river near the Turkish border, fighting was reported between rebels and soldiers around the Dalla roundabout and the centre for immigration and passports, the Observatory said.
The regime army launched air strikes on Raqa's central prison which was seized at the weekend by the jihadist Al-Nusra Front and other rebel groups, who then set free hundreds of inmates, the watchdog said.
Prior to the conflict Raqa was home 240,000 people, but more than 800,000 people have moved there to escape the daily violence elsewhere in the strife-torn country.
Hundreds of Syrian troops and troops have been killed in recent weeks mainly in the battle for a police academy in the northern province of Aleppo, with insurgents seizing control of most of the complex, the Observatory said.
"On Sunday, the highest number of troops and rebels combined were killed since the start of the conflict in Syria," Observatory director Rami Abdel Rahman told AFP.
"We were able to document a death toll of 219 for fighters from both sides for Sunday alone, but we are certain the actual toll is even higher," he added.