President Barack Obama Wednesday imposed tough sanctions against Syrian President Bashar al-Assad and six other senior Syrian officials in an effort to stop the regime's fierce crackdown on protests, the U.S. Treasury Department said.

The sanctions also target two top Iranian officials whose unit was a "conduit for Iranian material support" to Syrian intelligence, according to a copy of the executive order issued by the White House.

Obama signed the order Wednesday, a move a senior administration official described as a "decisive step to increase pressure" on the Syrian government to end violence, intimidation and pressure "and begin transitioning to a democratic" process.

Condemning Syria's use of violence and intimidation against its people, the official said al-Assad "must put an end to the attacks on protesters, mass arrests and harassment" of citizens expressing rights and "must begin to introduce change."

The Syrian government has launched a clampdown on peaceful demonstrators since mid-March. The United Nations last week said as many as 850 people have died in the protests, and there have been thousands of arrests.

Along with al-Assad, the other senior Syrian officials are Farouk al-Shara, Prime Minister Adel Safar, Interior Minister General Mohammad Ibrahim al-Shaar, Defense Minister Ali Habib Mahmoud, the head of Syrian military Abdul Fatah Qudsiya and Mohammed Dib Zaitoun, head of Syrian military intelligence.

"As a result of this action, any property in the United States or in the possession or control of U.S. persons in which the individuals listed in the Annex have an interest is blocked, and U.S. persons are generally prohibited from engaging in transactions with them," the Treasury Department said in a statement detailing the steps.

The senior administration official said it's too early to estimate how much will be frozen.

"As I am sure you know, when we take actions obligatory for institutions to freeze, many institutions around the world might" do the same thing, the official said. "Amplifications around that may be significant."

Treasury also announced the designation of 10 individuals and entities under an executive order signed by Obama on April 29.

It targets Syrian officials and others "responsible for human rights abuses, including repression against the Syrian people, as well as a set of companies tied to Syrian corruption."

One is Hafiz Makhluf, a cousin of al-Assad and a senior official in the Syrian General Intelligence Directorate, the overarching civilian intelligence service in Syria.

"Makhluf was given a leading role in responding to protests in Syria, and was heavily involved in the Syrian regime's actions" in the city of Daraa, the official said, where many protesters have been killed.

Also designated were two Iranians: Qasem Soleimani, commander of the Iranian Islamic Revolutionary Guard Corps-Quds Force, which is called "the conduit for Iranian material support" to the Syrian General Intelligence Directorate, and the other is Mohsen Chizari, a senior IRGC-QF officer who serves as its commander of operations and training.

"It is a completely hypocritical act" of the Iranian government and the Quds force to "lend assistance in oppression" against those expressing human rights, the senior official said. "It is something we are concerned about," the official added.

Other agencies designated are the Syrian Military Intelligence, the Syrian National Security Bureau, which is an element of the ruling Baath party; and Syrian Air Force Intelligence.

All are said to be involved in the actions against demonstrators.

Air Force Intelligence personnel allegedly "fired tear gas and live ammunition to disperse crowds of demonstrators" in late April after noon prayers, the Treasury Department said. At least 43 people were killed.

- CNN's Elise Labott and Adam Levine contributed to this report