BEIRUT — Two suicide car bombers struck security compounds in Aleppo on Friday, killing 28 people, Syrian officials said, bringing significant violence for the first time to an industrial centre that has largely stood by President Bashar Assad during the 11-month uprising against his rule.

Anti-Assad activists denied any involvement and accused the regime of setting off the blasts to smear the opposition as government forces pummel rebels in one of their main strongholds, Homs. State media touted the bombings as proof the regime faces a campaign by terrorists, not a popular uprising.

The military stepped up its siege of Homs, where hundreds have reportedly been killed 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.

Troops and security forces opened fire on anti-regime protesters who streamed out of mosques after Friday prayers nationwide. At least 27 civilians were killed, according to the London-based Syrian Observatory for Human Rights.

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

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.

Security officials said suicide bombers in explosives-packed vehicles tried to smash through the entrances of both sites. At the barracks, 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. A head lay on the ground and security officials said it belonged to a suicide attacker.

State television cited the Health Ministry as saying 28 people were killed in the two blasts and 235 wounded. The dead included 11 security personnel killed at the barracks, 13 military personnel killed at the military intelligence building and four civilians, security officials said.

State TV blamed "terrorists." But anti-government activists accused the regime of setting off the 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 did not carry out the bombings.

"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 northern city of some 2 million people that is home to a prosperous business community and merchant class 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"' -- a denunciation of Russia's veto of a U.N. attempt to condemn Syria's crackdown.

Regime forces opened fire on the Aleppo protesters, killing at least seven, according to the Observatory. Another activist group, the Local Coordination Committees put the Aleppo toll at 12 and said 22 others had been killed nationwide. The figures could not be independently confirmed 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 U.N. 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 to block a U.N. resolution calling on Assad to leave power.

The king of Saudi Arabia, which has led Arab efforts against Assad, made his first public comments on Syria's crisis Friday. He denounced the Russian and Chinese vetoes, saying "the confidence of the world in the U.N. has undoubtedly been shaken."

"These countries don't rule the world, never. The world is ruled by wisdom, fairness, morals, and standing up to the aggressor," King Abdullah said in a televised speech. "We are living scary, scary days."

Russian Deputy Foreign Minister Sergei Ryabkov signalled Friday that Moscow would use its veto again 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.

A week ago, security forces launched a major assault on 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 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.