Twin car bomb attacks have struck Syria's second city of Aleppo, according to state TV, as up to 10,000 troops are reported to be massing on the outskirts of the besieged city of Homs.
The explosions, which reportedly killed 28 people and injured 235, suggest a significant shift in the country's ever-deteriorating situation.
Aleppo is Syria's commercial centre, where wealthier Sunni merchant classes have been long-time supporters of the Assad regime.
As with the December bombs in Damascus, Aleppo had not seen such an attack in decades - and the relative stability the city had enjoyed throughout months of violence elsewhere in the country has been shattered.
State TV blamed "armed terrorist gangs" for what it called "suicide bombs" targeting security compounds in Aleppo, saying they were proof the government was facing a violent enemy.
But Mohammed Abu Nasr, an Aleppo-based activist, claimed President Assad 's regime was behind the explosions and insisted the opposition would not carry out bombings in residential areas.
And Colonel Riad al As'ad, the leader of the Free Syrian Army (FSA) based in Turkey, denied responsibility for the attacks in Aleppo.
An FSA spokesman said: "This criminal regime is... carrying out bomb attacks in Aleppo to steer attention away from what it is doing in Homs, Zabadani and elsewhere."
Sky's foreign affairs editor Tim Marshall said both sides would continue to blame each other: "The opposition says that ... the state military intelligence attacks its own army and its own police, in order to prove to the population that the measures they are taking at the moment ... are necessary."
While tactically the decision to draw attention away from the ongoing crackdown in Homs might be a questionable tactic for the FSA, he said: "We do not know the truth of the matter."
Meanwhile, the offensive by Syrian troops aimed at crushing rebels in the battered city of Homs - where hundreds have died in the past week - has continued.
Soldiers, who have been bombarding Homs for the past six days, have reportedly made their first move on the ground to seize one of its neighbourhoods.
Troops backed by tanks pushed into the district of Inshaat and are thought to have gone house to house detaining people.
Inshaat is next to Baba Amr - an area targeted in a sustained assault by regime forces since Saturday where more than 400 people have been killed, activists say.
Sky News chief correspondent Stuart Ramsay said families in Homs have been making preparations to die ahead of an anticipated major offensive by government forces in the city and towards the Lebanese border.
Ramsay said people were saying goodbye to loved ones as villages were being abandoned.
He said the FSA believes there is going to be a sweep by the regime's troops through the countryside to the border.
Ramsay, who has just left the country and is now in Lebanon, said the area was "quite a hotbed because the FSA and anti-government protesters can nip across the border fairly easily for safety".
He said: "The view is that the government is about to push through lots of towns and villages all the way to the border and sending with the troops the militia who will kill indiscriminately without any doubt.
"The feeling from most people is that it is not just about controlling Homs, it is about crushing this uprising in all the major cities where this is happening and that means killing everyone involved. It's that bad."
He added that the FSA was trying to fight back across the region against the regime but described it as "like a hornet on an elephant".
The FSA soldiers are understood to be running low on ammunition after six days of fighting.
As fears of a full-blown humanitarian crisis take hold and the international community considers its next move, there are concerns the regime's attention may be shifting to Idlib and now to Aleppo.
United Nations general secretary Ban Ki-moon has condemned Mr Assad's crackdown as "appalling brutality" and mooted the idea of a joint UN-Arab League monitoring mission.
Arab League foreign ministers are due to meet in Cairo, where the proposal - originally made by its chief Nabil Elaraby - is set to be discussed.
So far, diplomats and ministers including British Foreign Secretary William Hague have criticised the Syrian government but have ruled out military intervention. Mr Hague said Britain has no plans to arm the rebels.
Russia has given the Assad regime its backing despite fierce criticism from the West after Moscow and Beijing vetoed a UN Security Council resolution aimed at ending the violence.
Following a visit by its foreign minister Sergei Lavrov on Tuesday, it has now said no one should interfere in the country's affairs.
And deputy foreign minister Sergei Ryabkov signalled that Moscow will again use its veto power at the UN to block any resolution aimed at ousting President al Assad from power.
Mr Ryabkov accused the West of being an "accomplice" to the violence in Syria and said the country's opposition bore full responsibility for ending the violence.
But there is growing disquiet at the loss of life in Homs, where witnesses say makeshift hospitals are overflowing with the dead and wounded.
Medical supplies and food are also now running out.
A Syrian doctor, struggling to treat the wounded at a field clinic in a mosque, delivered an emotional plea on YouTube urging other countries to step in.
"We appeal to the international community to help us transport the wounded. We wait for them here to die in mosques. I appeal to the United Nations and to international humanitarian organisations to stop the rockets from being fired on us," he said.
The US has said it is considering ways to provide food and medicine to residents but even such a move would raise serious questions about the extent of international involvement.
The Kremlin, for whom Syria is a buyer of arms and host to a Soviet-era naval base, wants to counter US influence and maintain its traditional role in the Middle East.
His comments came after Russian Prime Minister Vladimir Putin said on Wednesday: "Help them, advise them, limit, for instance, their ability to use weapons but not interfere under any circumstances."
International officials have estimated the overall death count in Syria since last March at more than 5,000.
Bombardments in Homs on Thursday hit largely Sunni Muslim neighbourhoods. The main street in Baba Amr was strewn with rubble and bodies were pulled from the ruins.
Hussein Nader, an activist, said: "Whole houses have come down and we do not know how many more have been killed. They are not advancing and it seems that they are content by continuing to shell Baba Amr until every inhabitant is killed."
The uprising against the Assad family's 42-year dynastic rule has evolved from civilian demonstrations to armed insurgency in the past few months.
The regime insists it is fighting foreign-backed "armed terrorists".
The escalating violence in Syria may have left up to 6,000 dead since protests began 11 months ago, according to the UN.
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