DAMASCUS // Explosions rocked Syria's second city of Aleppo as tanks entered the protest hub of Homs on Friday, activists said, ahead of planned protests to denounce Russia's steadfast backing of the regime.
Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, said there were three blasts, one of them near a military intelligence building in the northern city.
State television said two explosions had taken place and reported a number of casualties, including soldiers. It blamed the attack on "armed terrorist gangs."
Protests have been largely muted so far in Aleppo, a main commercial hub.
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 opponents of President Bashar al-Assad, said Abdel Rahman.
"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 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.
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.
Rights groups estimate more than 6,000 people have died in a fierce government crackdown to put down a revolt that erupted nearly a year ago.
US President Barack Obama 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."
Troops trying to crush opponents of Assad have killed at least 400 people in a six-day onslaught on Homs city, opposition activists say.
More than 80 people were killed across Syria on Thursday alone, all but 30 of them in Homs.
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.
He said a joint Arab League-UN observer mission and the naming of a UN special representative on the Syria conflict "would serve peace and balance. And both would be a clear signal by the international community to the Assad regime."
EU foreign affairs chief Catherine Ashton said Russia must confront "the reality" of the deadly crackdown in Syria, after Moscow vetoed a UN resolution condemning the violence.
"My message to my Russian colleagues is they too need to recognise the reality of the situation on the ground" in Syria, said Ashton, who was making a two-day visit to Mexico.
Moscow has insisted that any solution to end nearly one year of bloodshed must come from within Syria.