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 of the casualties.
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.
In the northern city of Raqa 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.
Regime forces also 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 to 240,000 people, but more than 800,000 people have moved there to escape the daily violence elsewhere in the strife-torn country.
Scores of Syrian troops and rebels have been killed in the past few weeks as the fighting intensified, 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."
In the northern city of Aleppo, meanwhile, the army tried to take back the historic Umayyad mosque, days after rebels seized it, said the Observatory.
It added that in Aleppo province, insurgents pressed their efforts to capture airports, launching an assault on Minnegh air base, and blowing up a bridge to stop military reinforcements from reaching Aleppo international airport.
Meanwhile, the Local Coordination Committees, a grassroots network of activists, accused Iraq and Lebanese Shiite movement Hezbollah of "providing direct assistance to the Assad regime's army (by) bombarding rebel positions" near Syria's borders with Iraq and Lebanon.
A key Syrian opposition group on Sunday accused the government in Baghdad of intervening in the country and "attacking the Syrian people," a day after clashes were reported near the border.
Baghdad has pointedly avoided calling for Assad's departure.
Hezbollah has denied any role in Syria's nearly two year conflict.