Two suicide car bombers killed 28 and injured 235 people in Aleppo early on Friday, Syrian state media reported.

Security officials said suicide bombers tried to smash through the entrances of security headquarters with explosive-packed vehicles.

The explosions were the fourth dramatic suicide attack since late December, and the first significant violence in Aleppo, the most populated city in Syria.

Like previous instances, the bombings were aimed at various security headquarters. So far, one has claimed responsibility for the attacks. The explosions struck the local headquarters of the Military Intelligence Directorate and a barracks of the Security Preservation forces. Security officials said among the dead were 11 security personnel at the barracks, 13 military personnel at the military intelligence building and four civilians.

State media attributed the bombings to the work of “terrorists,” but anti-government activists accused the regime of setting the blasts to avert protests that were planned for Friday.

The bombings follow the continuation of assults in the city of Hom, where activists claim the regime has claimed hundreds of lives.

From New York Times:

“In Turkey, Capt. Ammar al-Wawi, a spokesman for the Free Syria Army, an opposition group of military defectors, denied involvement in the latest explosions.

He blamed the government itself for carrying out the attacks against what he said were two heavily guarded security compounds that it would have been difficult for civilians to approach.

“This regime is playing a well-known game, seeking to distract the world’s attention from the massacres in Homs,” Captain Wawi said. Another Free Syria Army commander said the rebels had carried out an operation against security headquarters before the bombings, but that they had nothing to do with the blasts.”

Assad’s crackdown on dissent has left the country almost isolated internationally, except for key support from Russia and China.

From the Associated Press:

“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 signaled 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 defense 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."

Assad's ruling Baath party is scheduled to hold its first conference since 2005 on Feb 17 after promising to calm the uprising.

Follow reporter Rosa Trieu on Twitter.