The sanctions put on the Syrian companies will freeze any assets the firms have in U.S. jurisdictions and will stop Americans from doing business with them. (Jason Reed/Reuters)

Ratcheting up the pressure, the Obama administration slapped sanctions Wednesday on Syria's largest commercial bank and cellphone operator as it moved to demand the end of four decades of dictatorship under the Assad family.

The Treasury Department added the state-owned Commercial Bank of Syria and its Lebanon-based subsidiary, the Syrian Lebanese Commercial Bank, to its sanctions list, citing their links to human rights abuses and to illegal weapons trade with North Korea.

Mobile phone company Syriatel was targeted because it is controlled by "one of the regime's most corrupt insiders," said David Cohen, the department's undersecretary for terrorism and financial intelligence.

The action freezes any assets the firms have in U.S. jurisdictions and bans Americans from doing business with them. The sanctions may not have much immediate economic impact because the U.S. already severely limits trade and economic ties with Syria, but they can further hamper the reputation of the companies and lead other governments, such as those in Europe, to enact tougher punishments on individuals and companies close to President Bashar Assad.

"We are taking aim at the financial infrastructure that is helping provide support to Assad and his regime's illicit activities," Cohen said in a statement.

The administration is also preparing for the first time to explicitly call for Assad to step down, officials have told The Associated Press, speaking on condition of anonymity to discuss internal administration deliberations.

The moves are a direct response to Assad's decision to step up the ruthlessness of his government's crackdown against pro-reform demonstrators by sending tanks into opposition hotbeds.

'Lost legitimacy'

President Barack Obama and other top U.S. officials previously have said Assad has "lost legitimacy" as a leader and that he either had to spearhead a transition to democracy or get out of the way. They had not specifically demanded that he step down. The new formulation will make it clear that Assad can no longer be a credible reformist and should leave power, the officials said.

White House spokesman Jay Carney said Wednesday the U.S. would continue to step up the pressure on Assad's regime through sanctions and other measures. "The most important thing we can do is make sure that our actions back up our words," he told reporters.  

Cohen said the Damascus-based Commercial Bank of Syria is an agent for Syrian and North Korean weapons proliferators.

The company, with 50 branches, provides financial services to the government research centre that oversees Syria's missile production facilities and weapons of mass destruction programs, according to Treasury.

It also holds an account for the North Korean bank Tanchon, which helps that country exports weapons and equipment related to ballistic missiles. Treasury said the bank also deals with several Iranian banks under U.S. sanctions. 

The department also issued an advisory alerting all U.S. financial institutions the Syrian bank is involved in money laundering.

Syriatel was targeted for being controlled by Assad's cousin, Rami Makhluf, a "powerful Syrian businessman and regime insider" who has faced U.S. sanctions since 2008 for benefiting from and aiding government corruption. Treasury said Makhluf has tried to hide his controlling interest in the company, but he continues to own and run the telecommunications firm.