Two suicide car bombers struck Syrian security compounds in Aleppo on Friday, killing 28 people, Syrian officials said, bringing significant violence for the first time to a major city that has largely stood by President Bashar al-Assad in the 11-month-old uprising against his rule.

State media touted the blasts as proof that the regime faces a campaign by terrorists, not a popular uprising against Assad's rule. The opposition, in turn, accused the regime of trying to smear its movement as government forces try to crush rebels in one of their main strongholds, Homs.

The military, meanwhile, stepped up its siege of Homs that has reportedly killed hundreds over the past week. Soldiers who have been bombarding the central city made their first ground move, storming into one of the most restive neighbourhoods.

At the same time, troops and security forces opened fire on anti-regime protesters who streamed out of mosques after Friday prayers nationwide. The Britain-based Syrian Observatory for Human Rights said at least 27 civilians were killed.

The morning blasts in the northern city of Aleppo, Syria's most populous, ripped apart the facades of the local headquarters of the Military Intelligence Directorate and a barracks of the Security Preservation forces in another part of the city.

At the Directorate, windows were shattered and a large crater was torn into the pavement outside the entrance. A weeping correspondent on state-run TV showed graphic footage of at least five corpses, collected in sacks and under blankets by the side of the road.

At both sites, suicide bombers in explosives-packed vehicles tried to smash into the entrances, security officials said. At the barracks, the Security Preservation forces commander Brig. Firas Abbas told an Associated Press reporter on a government-guided visit to the scene that the vehicle made it through one roadblock before detonating near the gates.

State television cited the Health Ministry as saying 28 people were killed in the two blasts and 235 wounded, including civilians and military personnel. It didn't give a breakdown of the individual casualty toll for each blast.

State TV blamed "terrorists." Anti-Assad activists accused the regime of setting off Friday's blasts to discredit the opposition and avert protests that had been planned in the city on Friday.

Capt. Ammar al-Wawi of the Free Syrian Army, a rebel group that wants to bring down the regime by force, denied involvement. He said fighters from his group had a short gunbattle with troops several hundred yards (meters) from the Directorate about an hour before the explosion but they did not carry out the bombings.

4th suicide attack since December

"This explosion is the work of the regime to divert world attention from the crimes it is committing against the people of Homs," he said.

The blasts were the fourth such dramatic suicide attack since late December. All occurred on Friday mornings against various security headquarters and prompted the same exchange of accusations. The earlier attacks, in the capital Damascus, killed dozens of security forces and civilians, according to Syrian officials. Nobody has claimed responsibility for any of the attacks.

Friday's bombings were the first significant violence in Aleppo, a city of some two million people that is home to a prosperous business community and merchant classes whose continued backing for Assad has been crucial in bolstering his regime.

The city has seen only occasional protests. Assad's opponents have had little success in galvanizing support there, in part because business leaders have long traded political freedoms for economic privileges. Also, the city has a large population of Kurds, who have mostly stayed on the sidelines of the uprising since Assad's regime began giving them long-denied citizenship as a gesture to win support.

Still, hours after the explosions, hundreds of protesters marched in several Aleppo neighbourhoods after Friday prayers, part of nationwide demonstrations labeled "Friday of 'Russia is killing our children"' — denouncing Russia's veto last weekend of a UN attempt to condemn Syria's crackdown.

Regime isolated internationally

Regime forces opened fire on the Aleppo protesters, killing at least seven, according to the Observatory. Another activist group, the Local Co-ordination Committee put the Aleppo toll at 12 and said 22 others had been killed nationwide. The figures could not be independently confirmed, in part due to restrictions the Syrian government has put on journalists.

Assad's crackdown has killed well over 5,400 people since the uprising began in March, according to UN estimates.

The regime's crackdown on dissent has left it almost completely isolated internationally — except for key support from Russia and China, which delivered a double veto last Saturday to block a UN resolution calling on him to leave power.

Russian Deputy Foreign Minister Sergei Ryabkov signalled Friday that Moscow will again use its veto power at the United Nations to block any resolution aimed at ousting Assad.

"If our foreign partners don't understand that, we will have to use strong means again and again to call them back to reality," he was quoted as saying by the ITAR-Tass news agency.

Moscow's stance is motivated in part by its strategic and defence ties, including weapons sales, with Syria. Russia also rejects what it sees as a world order dominated by the U.S. Last month, Russia reportedly signed a $550 million deal to sell combat jets to Syria.

Across Syria on Friday, thousands held protests denouncing the Russian position, from the northwestern province of Idlib, to the suburbs of Damascus, the Mediterranean coastal city of Latakia and the eastern town of Deir al-Zour.

Neighbourhoods bombarded Friday

A week ago, security forces launched a major assault on the central city of Homs after unconfirmed reports that army defectors and other armed opponents of Assad were setting up their own checkpoints and taking control of the most restive neighbourhoods.

Days of bombardment of the neighbourhoods with artillery, heavy machine-guns and mortars continued on Friday, as troops on the ground backed by tanks for the first time pushed into one of the districts, Inshaat, activists said.

The Observatory said troops were going house to house detaining people. Inshaat is next to Baba Amr, a neighbourhood that has been under rebel control for months.

"They are punishing the residents," said the Observatory's chief Rami Abdul-Rahman, who added that food supplies were dwindling in the area.

Mohammed Saleh, a Syria-based activist, said the regime appears to be trying to take over rebel-held areas in Homs and Idlib before Feb. 17, when Assad's ruling Baath party is scheduled to hold its first general conference since 2005.

The conference is expected to move on reforms that Assad has promised in a bid to calm the uprising. During the conference, Baath party leaders are expected to call for national dialogue and announce they will open the way for other political parties to play a bigger role in Syria's politics.

The opposition has rejected such promises as insincere and said it will not accept anything less than Assad's departure.