Opposition denies government claims that it was behind bomb attacks in northern city that left dozens of people dead.
Syria's opposition has denied claims by the government that it was behind bomb attacks in the northern city of Aleppo that left dozens of people dead.
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 RPGs and light weapons," he said.
Syrian state television quoted the health ministry as saying that 28 people were killed and more than 200 others wounded, including soldiers and civilians.
"The number of casualties from the two car bombs in Aleppo has risen to 28 dead and 235 wounded," the ministry said.
The broadcaster blamed the attack on "armed terrorist gangs".
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.
He said the security buildings were heavily guarded by security forces and that it would be "impossible" to carry out Friday's blasts.
The UK-based Syrian Observatory for Human Rights said 30 people were killed in the explosions in the neighbourhoods of Sakhur and Marjeh and the Dawar el-Basel roundabout.
Syria blamed the blasts on "terrorists" backed by Arab and Western nations, in a letter sent to the UN secretary general, the UN Security Council, the Arab League and other organisations.
"Certain countries in the region are behind a propaganda campaign against Syria and are harbouring armed terrorists for so-called humanitarian reasons," it wrote, the official SANA state news agency reported.
Three deadly bombings aimed at security targets occurred in the capital, Damascus, in December and January. The government blamed al-Qaeda, while the opposition accused the authorities of carrying out the attacks.
Friday's bombings were the first significant violence in Aleppo, a city of about two million people that is home to a prosperous business community which has stayed largely loyal to the government.
The city has seen only occasional protests, but hours after the explosions hundreds of protesters marched in several neighbourhoods after Friday prayers.
Security forces opened fire on the protesters, killing at least six people, according to the Local Co-ordination Committees.
The activist network said 12 people were killed in the province and at least 23 more in other parts of the country.
'Russia killing our children'
Demonstrations were held in cities across Syria on what activists called, "Russia is killing our children" Friday, in response to Russia and China vetoing a UN Security Council resolution last week.
Moscow said Syria's opposition "bears full responsibility" for the ongoing violence, while accusing the West of pushing regime opponents into armed conflict.
Arab and Western governments have so far resisted mounting calls for military aid to the outgunned and outnumbered rebels in Syria.
In central Syria, tanks stormed the Inshaat district in Homs overnight as troops launched a house-to-house sweep to crush opposition members, Rami Abdul-Rahman, director of the UK-based Syrian Observatory for Human Rights, said.
Inshaat is next to Baba Amro district, which has been subjected to a withering assault by regime forces since last Saturday that has killed more than 450 people, activists say.
In addition to the victims of the attacks in Aleppo, the Observatory said another 44 people were killed across Syria on Friday - 28 civilians, nine soldiers and seven deserters.
Security forces deployed heavily outside mosques nationwide, firing on worshippers in some areas to prevent protests denouncing Russia's steadfast support for the Assad regime, activists said.
"Demonstrations broke out in various parts of the country, but they were small in numbers because of the heavy deployment of army and security forces as well as the cold weather," Abdel Rahman 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'
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.
In the latest example of how the crisis in Syria is affecting neighbouring Lebanon, at least five person was reportedly wounded in gunfire and grenade blasts in the Lebanese city of Tripoli, where Sunni Muslim and Alawite communities are divided over the Syrian issue.
Armed men have deployed heavily in two rival neighbourhoods, one group supporting, and the other opposed to, President Bashar al-Assad's leadership in Syria.
"There is a heavy armed presence and shooting in the Sunni Muslim neighbourhood of Bab al-Tebbaneh and the Alawite neighbourhood of Jabal Mohsen," a security official said.
He said the army had deployed in both neighbourhoods, but later retreated to a street dividing the two sides. Five people were reported wounded, including two soldiers.