A bomb blast at a security building in Damascus on Wednesday killed at least three top Syrian officials in what some experts are calling a game-changer in the 16-month uprising.

Update 5:08 p.m.: Unconfirmed reports say Free Syrian Army is on the move in Damascus

After more than half a day of discussion online, rumors continue to flood the Web regarding the Free Syrian Army’s movements in Damascus.

Rami Jarrah, who tweets under the moniker of Alexander Page from Egypt, reported that the Free Syrian Army was marching on the Syrian presidential palace.

Please spread to friends and family in #Syria incase of exposure to poisonous gases rap some coal in a wet cloth and cover nose and mouth

— Alexander Page (@AlexanderPageSY) July 18, 2012

Jarrah, who says he’s fed information from sources inside Syria, also warned his followers that the government could use poisonous gas to quell the uprising.

Please spread to friends and family in #Syria incase of exposure to poisonous gases rap some coal in a wet cloth and cover nose and mouth

— Alexander Page (@AlexanderPageSY) July 18, 2012

Robert Mackey of the New York Times’ The Lede blog put together a list of videos posted by activists on YouTube showing Wednesday’s violence in Damascus.

Update 4:30 p.m.: Homs activist reacts to deaths of top Syrian officials

A Syrian activist, who calls himself Abu Emad, describes the reaction in the city of Homs to the death of top Syrian regime officials in Damascus. He says heavy shelling in Homs followed the defection of hundreds of soldiers upon hearing the news.

Update 3:13 p.m.: Syrian troops shooting in Damascus:

A video released by the Telegraph via the Syrian news agency, SANA, purports to show Syrian government troops shooting in a Damascus neighborhood shortly after this morning’s attack.

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Update 2:30 p.m.: Obama tells Russia’s Putin that siding with Assad puts him on “wrong side of history”

During his daily briefing, White House press secretary Jay Carney said President Obama and Russian President Vladimir Putin talked by phone this morning about the situation in Syria and that the administration stressed that allying with the Assad regime would put Russia on the "wrong side of history," reports The Post’s David Nakamura.

President Obama met with his Russian counterpart, Vladimir Putin, in Los Cabos, Mexico, in June during the G20 leaders summit. (ALEXEI NIKOLSKY/AFP/GETTY IMAGES)

Update 1:30 p.m.: Militiamen killing families with knives

The Post’s Liz Sly writes: “Damascus residents reported that pro-government shabiha militiamen were swarming into the streets of several Damascus neighborhoods bent on exacting revenge. According to one eyewitness in the Shaghour neighborhood of the historic walled Old City, militiamen were breaking down doorways and killing families with knives.

The man, who asked not to be identified, said he was watching scenes of panic from the roof of his building, as both men and children, carrying guns and knives, ran onto the street to try to defend the area against the militias. Activist Tareq Saleh of the Revolutionary Leadership Council of Damascus said he was receiving reports of similar killings in the areas of Hajar al-Aswad and Qadam.

"Shabiha militias are killing people with knives," he said. "There are tens of bodies on the streets."

The information was impossible to independently confirm, but activists posted videos of what appeared to be three bodies in the streets of Qadam.

"The situation is getting worse by the second," said a Qaboun resident contacted by Skype who asked not to be identified because he fears for his safety.”

Armed people are walking in the streets. I can't tell who they are or what God they believe in."I saw them and I stayed away from the window scared that they might see me."

Updated 1:23 p.m.: U.N. postpones vote

“The U.N. Security Council has delayed a vote on a new Syria resolution until Thursday in a last-minute effort to get key Western nations and Russia to reach agreement on measures to end the dramatically escalating violence,” the AP reported.

Updated 1:15 p.m.:Shabiha shooting residents in Damascus — CNN

CNN reports that, “the pro-government Shabiha militia took to the streets of Damascus, shooting some and saying, ‘This is retribution for what you have done,’ according to an opposition activist in Damascus.

Updated 12:50 p.m.: Reports of civilian militia rampage

The Post’s Liz Sly wrote on Twitter:

Some terrifying reports of killings on streets of Damascus. Shabiha on the rampage w knives, guns.Residents taking 2 the streets to defend.

— Liz Sly (@LizSly) July 18, 2012

Updated 12:17 p.m.: National Security Council says Assad is losing control

U.S. National Security Council spokesman Tommy Vietor said: “It’s clear that the Assad regime is losing control of Syria. There is real momentum against Assad, with increasing defections, and a strengthened and more united opposition that is operating across the country. Many formerly pro-regime Syrians view Assad as the problem, not the solution, and the regime’s financial struggle continues.”

Vietor said the United States and international partners will continue to push for new leadership by supporting the Syrian rebels and keeping sanctions on President Bashar al-Assad.

Updated 11:55 a.m.: Russian foreign ministry condemns the bombing

“Moscow strongly condemns all forms and manifestations of terrorism. We hope the masterminds of the terrorist attack in Damascus will be found and brought to justice,” Russian Foreign Ministry spokesman Alexander Lukashevich said on Wednesday.

Earlier today:

A bomb blast at a security building in Damascus on Wednesday killed at least three top Syrian officials in what some experts are calling a game-changer in the 16-month uprising.

The bombing killed Defense Minister Dawoud Rajha; his deputy, Asef Shawkat, a former military intelligence chief who was married to Assad’s sister; and Hassan Turkmani, the head of the security cell.

The opposition Free Syrian Army has claimed responsibility for the attack, and there are reports on Twitter of government soldiers defecting.

Last week, a special correspondent for The Washington Post reported that the Syrian revolution was encroaching on the capital and challenging President Bashar al-Assad’s hold on power there.

Earlier reports that the attack was a suicide bombing appear to be false.

The Washington Post reports that the rebel Free Syrian Army said it planted bombs inside a room where government security chiefs were to meet and blew up the bombs when the meeting was underway.

Al-Jazeera has a good livestream from Damascus:

Anti-government activists are reportedly celebrating the deaths in the streets of Jarjanaz, according to the Guardian.

Turkish Prime Minister Recep Tayyip Erdogan was in Moscow to attempt to persuade Russian President Vladi­mir Putin to harden the country’s stance on Syria:

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