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The Obama administration on Friday imposed financial penalties against three top Syrian officials, Syria's intelligence agency and Iran's Revolutionary Guard over the violent crackdown on demonstrators in Syria.

The penalties freeze any assets that the officials, including two relatives of Syrian President Bashar Assad, as well as Syria's General Intelligence Directorate and the Qods Force of the Iranian Revolutionary Guard Corps may have in U.S. jurisdictions and bar Americans from doing business with them.

President Barack Obama authorized the action for repression and human rights abuses committed by Syrian authorities seeking to quell six weeks of protests. Activists say more than 500 people have been killed, 42 on Friday alone.

The White House said the penalties affect Maher Assad, Assad's brother and commander of the Syrian Army's Fourth Armored Division, which is accused of carrying out the worst atrocities in the southern city of Daraa; Assad cousin Atif Najib, the former head of the Political Security Directorate in Daraa Province; and intelligence chief Ali Mamluk.

"The United States strongly condemns the Syrian government's continued use of violence and intimidation against the Syrian people," according to the U.S. statement. "We call upon the Syrian regime and its supporters to refrain from further acts of violence and other human rights abuses against Syrian citizens seeking to express their political aspirations."

"We call on President Assad to change course now, and heed the calls of his own people," White House spokesman Jay Carney said in a statement.

The penalties probably will have limited direct impact because none of the targets is believed to have any significant assets in U.S. banks. But officials said the move was aimed at sending a clear message to the Syrian people that those responsible for the crackdown are going to face consequences and no one in the Syrian leadership will be immune.

Secretary of State Hillary Rodham Clinton described the Syrian government's actions against its own people as "absolutely deplorable." She said the sanctions were intended to show the government it would be held to account.

She welcomed the U.N. Human Rights Council's condemnation of Syria for its violent crackdown and for mandating a U.N.-led investigation into alleged violations of international law with the goal of bringing perpetrators to justice. She said the U.S. supported that action and would strongly support its implementation.

Although Assad himself is not among those mentioned Friday, officials said he could be named at a later date if the crackdown continues. In a related move, the Commerce Department said it was revoking licenses for the export to Syria of items relating to VIP aircraft used to transport senior Syrian officials.

Sen. John Kerry, the chairman of the powerful Senate Foreign Relations Committee who had advocated greater engagement with Syria, welcomed the sanctions. He said they were part of necessary pressure on Assad to ensure he "understands that he must end the violence and embrace reforms."

"The administration's imposition of tough economic sanctions against the perpetrators of these grave human rights abuses is appropriate. It puts Syria's leaders on notice that decisions to kill unarmed civilians have consequences," he said in a statement.

Syria is already under U.S. sanctions because it's designated a "state sponsor of terrorism" by the State Department. The new ones extend the penalties to individuals.

Iran and its Revolutionary Guard Corps are under similar U.S. punishment. The officials said the new designation for the Revolutionary Guard would add another layer of penalties and make clear that Washington believes it is providing material support to help Syrian authorities with the crackdown.

In addition to the new sanctions, Obama extended the national emergency as it relates to Syria, saying that its support for militant anti-Israel groups, ongoing pursuit of weapons of mass destruction and interference in neighboring Lebanon make it a "continuing unusual and extraordinary threat to the national security, foreign policy, and economy of the United States." The emergency was to have expired on May 11.

Friday's announcements came as Syrian security forces opened fire on demonstrators trying to break an army blockade on Daraa, while thousands across the country defied a protest ban and denounced Assad. At least 42 people were killed, including 15 in the march on Daraa, according to witnesses and a human rights group.

The U.N. Human Rights Council demanded that Syria stop the violence, release political prisoners and lift restrictions on the news media and access to the Internet.

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Associated Press writer Matthew Pennington contributed to this report.

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State Department background: http://www.state.gov/p/nea/ci/sy/