Syrian security forces have renewed their assaults on the central cities of Homs and Hama, while anti-government forces are said to have killed soldiers in Idlib and a senior military official in Damascus, activists, rights groups and the state news agency report.
Opposition neighbourhoods in Homs were hit by tank and rocket bombardments on Saturday in the government's continuing crackdown on protesters there, with the city's Bab Amr area coming under concerted fire.
Activists said that the neighbourhood came under a renewed assault late on Saturday night, ahead of a meeting of Arab foreign ministers due to discuss setting up a joint observer mission with the United Nations.
Ministers from the Arab League, which suspended Syria in response to the crackdown, meet in Cairo on Sunday to discuss forming a joint UN-Arab monitoring team in place of an observer mission that was suspended last month.
An official source at the League said another proposal called for the appointment of a UN-Arab League envoy to deal with Syria, adding that some states may also propose the Arab League formally recognise the opposition Syrian National Council.
Rights groups say at least four people were killed during Saturday's violence in Homs, and that three bodies of people who had been killed earlier were also recovered.
Security forces have also made house-to-house raids over the last two days in Homs, which has been under siege for the last week, anti-government activists have said.
They say at least 300 people have been killed there since the government's latest assault began on February 4.
In Hama, the Syrian Revolutionary General Commission (SRGC), a rights group that has organised protests against President Bashar al-Assad's rule, says that the army is continuing to maintain a heavy presence. The group said troops raided the city's al-Sharia neighbourhood and that heavy gunfire and mass arrests were reported from there and at least three other districts.
Reports of violence between security forces and anti-Assad forces were also reported from Zabadani and Douma, which is just outside the capital Damascus.
The SRGC said that at least 46 people were killed in Saturday's violence: 20 in Homs, 11 in Deraa, 11 outside Damascus, three in Idlib and one in Hama.
Elsewhere, the Syrian Observatory for Human Rights (SOHR), an UK-based rights group, reported that 10 soldiers were killed in an ambush by army defectors on Friday in the Idlib area, near the Turkish border. The defectors hit a patrol between two villages with hand grenades and roadside bombs, the SOHR said.
Army defectors also fought government troops for hours overnight in the al-Qaboun neighbourhood of Damascus.
Also in Damascus, state media reported that anti-government forces had killed a senior Syrian military doctor outside his home.
The SANA news agency reported that "an armed terrorist group" had killed Brigadier-General Issa al-Khouli, who it described as a doctor and hospital director, in the Rukneddine district of the Syrian capital on Saturday.
The Revolution Leadership Council, an anti-Assad group in Damascus, alleged that the government had carried out the killing itself.
Meanwhile, the armed opposition has denied claims from the government that it carried out bomb attacks in the northern city of Aleppo that left dozens of people dead on Friday.
Speaking to Al Jazeera, Arif al-Hummoud, a commander of the Free Syrian Army, a name used by various armed groups, said that opposition fighters had carried out an attack on Friday morning but were not responsible for the blasts.
"A group from the Free Syrian Army attacked a branch of the military security and a security unit in Aleppo with only [rocket-propelled grenades] and light weapons," he said.
In-depth coverage of escalating violence across Syria
Syrian state television quoted the health ministry as saying that 28 people were killed and more than 200 others wounded, including soldiers and civilians, in the attack and said it was carried out by "armed terrorist groups".
Abdul Rahman Abu Hothyfa, a spokesman for the opposition Syrian Revolution Co-ordinators Union, told Al Jazeera that it was "absolutely the regime" that carried out the attack.
Security forces opened fire on the protesters in several Aleppo neighbourhoods after Friday prayers, killing at least six people, according to the Local Co-ordination Committees, an umbrella organisation of anti-Assad activists.
The activist network said 12 people were killed in the province and at least 23 more in other parts of the country on Friday.
In addition to the victims of the attacks in Aleppo, the observatory said another 44 people were killed across Syria on Friday, including 28 civilians, nine soldiers and seven deserters.
Security forces deployed heavily outside mosques nationwide, firing on worshipers in some areas to prevent protests denouncing Russia's steadfast support for the Assad regime, activists said.
An activist in Homs giving her name as Salam al-Homsi said it was getting more difficult to save those injured.
"We can't even get them out of the rubble because the snipers won't let people help the wounded," she said. "There are no medical supplies and people are dying in the field hospitals. It's getting worse and worse."
'Destruction of Homs'
|Al Jazeera's Sue Turton reports from Tripoli where supporters and opponents of Assad are trading gunfire.
Robert Ford, the US ambassador to Syria, has posted declassified photographs on his Facebook site that Washington said shows soldiers attacking a civilian area of Homs.
State Department spokeswoman Victoria Nuland said the "declassified US national imagery" shows the destruction of Homs.
Nuland said Ford, who left Damascus when the US embassy was closed for security reasons on Monday, would continue to use Facebook to "talk to the Syrian people on a regular basis, along with his Twitter feed".
The photographs also apparently show sophisticated military hardware, including tanks, mortars and artillery targeting civilian areas.