US President George W Bush has approved economic and diplomatic sanctions against Syria over its alleged support for terrorism.
Syria has been told to end its support for terrorists
Possible actions include a ban on US exports to Syria and the freezing of the country's assets in the US.
BBC Washington correspondent Michael Buchanan says Mr Bush signed the Congress-inspired bill reluctantly.
The president hinted that he might waive some of the measures in the interests of national security.
"My approval of the act does not constitute my adoption of the various statements of policy in the act as US foreign policy," he said in a statement.
The bill - passed by large majorities in both chambers of Congress - is intended to penalise Syria for its alleged support for anti-Israeli militant groups and for its apparent pursuit of biological and chemical weapons.
It makes a series of demands on Syria, and requires the president to take action if these are not met.
Mr Bush will be required to ban sales of dual use technology, which could be used for both civilian and military purposes.
End support for terrorists
Pull troops out of Lebanon
End acquisition and production of weapons of mass destruction and long-range ballistic missiles
Prevent terrorists and weapons from entering Iraq
Ban on sales of dual-use technology
Prohibition of operations by US businesses
Limits on Syrian airline flights within US
Reduction of diplomatic contacts
Freeze on Syrian assets in US
He must also impose at least two of six possible penalties, which include flight restrictions on Syrian planes and the limitation of diplomatic contact between the two countries.
Our correspondent says the president was opposed to the bill, as it puts restraints on his diplomatic options, but changed his mind once he saw the strength of feeling in Congress.
The White House only agreed to the measure in October after Congress agreed to allow Mr Bush the option of waiving the sanctions in the interests of national security.
Administration officials says Damascus has made some progress recently, preventing anti-American elements from crossing into Iraq and supporting a US-backed United Nations Security Council resolution on Iraq.
Correspondents say that sanctions would have greater political than economic impact as trade between the US and Syria is only about $300m.
Syria is on the US State Department's list of state sponsors of terrorism, along with North Korea, Sudan, Cuba, Iran and Libya, but is the only one of these countries to have full diplomatic relations with Washington.