By Ran Wei, PNA / Xinhua and U.S. News Agency / Asian

U.S. President Barack Obama on Thursday for the first time explicitly urged Syrian President Bashar al-Assad to step down, imposing the harshest sanctions on the Middle East country to date.

In the tough-worded statement released by the White House, Obama said Assad’s calls for dialogue and reform “have rung hollow. ”

“We have consistently said that President Assad must lead a democratic transition or get out of the way,” he said, adding that “he has not led.”

“For the sake of the Syrian people, the time has come for President Assad to step aside,” said Obama.

Last month, the controversial visit by U.S. Ambassador Robert Ford to the central Syrian city of Hama triggered the attacks on the U.S. embassy and the ambassador’s residence in Damascus by some Assad loyalists.

Since then the two countries’ relationship has been in a free fall, during which both Obama and U.S. State Secretary Hillary Clinton had said that Assad “has lost legitimacy.”

Ted Carpenter, senior fellow with the Washington-based Cato Institute, told Xinhua that Obama’s move was widely expected.

“To the extent that foreign policy calculations played a role in the president’s decision, the two overriding goals were to show reformers in the Arab world that U.S. sympathies are with them and to back Saudi Arabia’s campaign to weaken Iran’s influence, since Assad is regarded as a junior ally of Tehran,” Carpenter said.

In announcing the “unprecedented” U.S. sanctions on Syria on Thursday, Obama said he issued a new executive order that immediately froze all assets of the Syrian government under U.S. jurisdiction, and prohibited all U.S. persons from engaging in any transaction involving the Syrian government.

The executive order also “bans U.S. imports of Syrian-origin petroleum or petroleum products; prohibits U.S. persons from having any dealings in or related to Syria’s petroleum or petroleum products; and prohibits U.S. persons from operating or investing in Syria,” he said in the statement.

Quickly following the Obama statement, the U.S. Treasury announced that it barred trade with five Syrian oil and gas companies, including General Petroleum Corporation, Syrian Company For Oil Transport, Syrian Gas Company, Syrian Petroleum Company and Sytrol.

However, experts believed the sanctions will have far more symbolic than substantive impact, given the facts that Syria’s economic exposure to the U.S. is minimal and the country is not a major energy producer.

Carpenter also warned that by taking the step to become involved in Syria’s violent internal political struggle, the Obama administration will come under increasing pressure to adopt more drastic measures, including leading a Libya-style NATO military intervention.

Syria has been in unrest since mid March when anti-government protests broke out in the southern province of Daraa and spread to other cities.

The Syrian authorities blamed the unrest on “armed groups and foreign conspiracy” and stressed that it would track down gunmen who have intimidated people and damaged public and private properties.