Syria government and rivals trade blame for Aleppo blasts
The Syrian government and opposition have blamed each other for two bombs targeting security compounds in the country's second city, Aleppo.
State television said at least 28 people were killed, including civilians and members of the security forces.
"Armed terrorist gangs" were responsible, according to state TV.
The rebel Free Syrian Army (FSA) said it had clashed with security forces in the area but that the government was behind the explosions.
Also on Friday, US Ambassador to Syria Robert Ford posted a note on the US Embassy in Damascus's Facebook page condemning "the use of heavy weaponry... against residential neighborhoods" in Homs, calling it "a new low for the Assad regime".
Private American satellite image provider DigitalGlobe Inc has released photos that it says show Syrian army activity on Friday in the city of Homs, which has been under heavy bombardment from government forces for days.
Stephen Wood, director of DigitalGlobe's analysis centre, told AP news agency said the images reveal an increase in the level of army activity in and around Homs from the previous 24 hours, with some showing armoured vehicles near apartment buildings.
Syrian state TV said one of Friday's bomb went off near a park in Aleppo and that children were among those killed.
The blast left a crater several metres wide in the road and blew a lorry onto its side.
But Col Malik al-Kurdi, the FSA's deputy leader, told BBC Arabic his soldiers had clashed with security forces personnel and members of the pro-government Shabiha militia outside a Military Intelligence compound and riot police base in Aleppo on Friday morning.
When they were gathering in a square to go to the mosques and repress demonstrations, two groups from the FSA targeted the two buildings with small arms and rocket-propelled grenade fire," he said.
"After violent clashes, there was an explosion inside the Military Intelligence building. We think it was the regime trying to stop the operation of the FSA."
Activist Izzedine al-Halabi told the BBC he held the regime "entirely responsible for this explosion".
Aleppo has seen only minor protests and relatively little violence since the uprising against President Bashar al-Assad erupted in March. Human rights groups say more than 7,000 civilians have been killed in the country.
Later on Friday, the Local Co-ordination Committees, an activist group that organises and documents protests, said another 51 people had been killed by security forces, across Syria, including 13 at a protest in the Marjeh district of Aleppo and 16 in Homs.
Activists say the intense bombardment of many parts of Homs by security forces since last Saturday - condemned as "outrageous bloodshed" by US President Barack Obama - has left more than 400 people dead.
Syria restricts access to foreign media and casualty figures cannot be independently verified.
The US closed its embassy in Damascus earlier this week and recalled Mr Ford and all American embassy employees from the country, saying the Syrian government had not done enough to guarantee their security.
In his Facebook post, Mr Ford said: "The artillery used by the regime is designed for full scale warfare, and the regime is using it to pound civilian apartment buildings and homes from a distance," citing satellite images allegedly showing Syrian army artillery deployments published by the State Department.
"We are intent on exposing the regime's brutal tactics for the world to see," he said.
"Some try to equate the violence perpetrated by the regime with the violence perpetrated by the opposition - it is unfair to do so when one side is using such heavy weaponry."
Residents of Homs said on Friday that tanks were massing outside several opposition-held districts.
Tanks had entered the district of Inshaat, next to the protest centre of Baba Amr, the Syrian Observatory for Human Rights said.
There was sporadic shelling and gunfire throughout the city on Friday.
There were calls for nationwide protests on Friday to denounce Russia and China's veto last week of a UN Security Council resolution backing an Arab League plan calling on President Assad to hand over power to his deputy.
But activists said heavy military deployment and cold weather meant the turnout was low.
Russia's Deputy Foreign Minister Sergei Ryabkov said the opposition "bore full responsibility" because it had refused to begin talks with the government and accused Western powers of being "accomplices".
Saudi Arabia - which has has been critical of Russia's support for Mr Assad - is circulating a draft UN General Assembly resolution similar to the one which was vetoed. The General Assembly, in which no state has a veto, is to discuss Syria on Monday.
The Saudi text calls on all parties in Syria to stop the violence, including armed groups, but places most of the blame on the government. The draft also adds a request for the Secretary General to appoint a special envoy to promote a peaceful solution to the crisis.