Two powerful car bombs targeting security posts ripped through Syria's second city of Aleppo on Friday, killing at least 25 people and wounding 175, even as tanks surged into battered protest hub Homs.
State television said "armed terrorist groups" carried out the attacks, the first in Aleppo since the outbreak of an uprising against the regime of President Bashar al-Assad almost a year ago.
It said a "suicide bomber in a car packed with explosives" carried out one of the attacks on a police station, flattening a nearby food distribution centre. The second bombing targeted an intelligence base.
The report showed mangled bodies in pools of blood in the street outside rows of shattered buildings, and deep craters in the ground.
Emergency workers held up body parts, including hands, feet and a torso which they placed in black trash bags.
"The number of martyrs who have been transported to hospital in Aleppo have so far reached 25 dead and 175 wounded as a result of the terrorist attacks," the health ministry said, as quoted by state television.
Aleppo, a northern commercial hub, has been largely spared the unrest that has rocked Syria since last March, leaving more than 6,000 people dead according to rights groups.
In central Syria, tanks stormed a district in the flashpoint city of Homs as troops launched a house-to-house sweep of the area to crush the Assad regime's opponents, said Rami Abdel Rahman of the Syrian Observatory for Human Rights.
"The tanks entered the neighbourhood of Inshaat overnight," he said, adding troops were still deploying.
Inshaat is next to the protest hub of Baba Amr in Homs, which has been subjected to a withering assault by regime forces since Saturday that has killed more than 400 people, activists say.
The violence escalated ahead of planned nationwide protests Friday to denounce Russia's steadfast support of the Assad regime.
"Russia is killing our children. Its planes, tanks and veto are also killing our children," said a banner on the Facebook page of The Syrian Revolution 2011.
Russia hit back on Friday, saying the opposition bore full responsibility for the ongoing violence while accusing the West of pushing the regime's opponents into armed conflict.
Deputy Foreign Minister Sergei Ryabkov told ITAR-Tass news agency the opposition's refusal to enter direct talks with the Syrian government meant it "bears full responsibility for improving the situation."
He accused the West of being "accomplices in the process of inflaming the crisis."
Moscow, a staunch ally of Assad along with China, has stood in the way of repeated attempts at the United Nations to adopt a resolution condemning the bloodshed in Syria.
US President Barack Obama on Thursday decried the violence as "outrageous bloodshed", in comments after White House talks with Italian Prime Minister Mario Monti.
He urged "a transition from the current government that has been assaulting its people."
British Prime Minister David Cameron said Assad's regime appeared determined to kill its own people.
"It's quite clear that this is a regime that is hell-bent on killing, murdering and maiming its own citizens," Cameron told reporters in Stockholm. "It really is appalling, the scenes of destruction in Homs."
He called for "transition and change in Syria."
Despite mounting calls for military aid to outgunned and outnumbered rebel troops in Syria, Foreign Secretary William Hague stressed Britain has no such plans.
But Turkish Foreign Minister Ahmet Davutoglu said the international community cannot afford to watch the "massacre" taking place in Syria without acting.
"We cannot let Syrian people die every day and the international community will follow blindly," Davutoglu said in Washington.
Germany meanwhile backed a proposed joint Arab League-UN mission to monitor the situation in Syria, but other major powers were more cautious.
Prospects for the mission that the pan-Arab bloc's chief has proposed to UN leader Ban Ki-moon could depend on an Arab League foreign ministers' meeting this weekend and the backing of the major powers.
As the international community struggles to find a new diplomatic response to Assad's assault on protest cities, Germany's Foreign Minister Guido Westerwelle gave the strongest welcome to the Arab League-UN proposal.
"In addition to the establishment of a contact group of 'friends of a democratic Syria' we must also undertake a new attempt to resolve the crisis through the United Nations," Westerwelle said in Berlin.
Moscow has insisted that any solution to end nearly one year of bloodshed must come from within Syria.