President George Bush in a statement on Tuesday said the sanctions were in response to "threats" posed by Syria to the national security, foreign policy and economy of the United States.
Bush accused Syria of "supporting terrorism, continuing its occupation of Lebanon, pursuing weapons of mass destruction and missile programmes, and undermining US and international efforts with respect to the stabilisation and reconstruction of Iraq."
"Syria's actions constitute an unusual and extraordinary threat to the national security, foreign policy, and economy of the United States and hereby declare a national emergency to deal with that threat," Bush said in the statement.
The export ban and other measures on Syria also follow long-standing US complaints that the Middle Eastern country had failed to stop anti-occupation fighters from entering neighbouring Iraq.
US bilateral trade with Syria is about $300 million a year.
A State Department official earlier during the week said that exemptions would be made to any export restrictions to allow the sale of aircraft spare parts, so that Syrian planes are not endangered and to allow the sale of communications equipment, to help Syrians get access to outside information.
The sanctions include a ban on flights to and fro from the US, authorisation to the Treasury Department to freeze assets of Syrian nationals involved in "terrorism" and restrictions on banking relations between US banks and the Syrian national bank.
The sanctions go beyond the minimum requirements of the Syrian Accountability Act which Bush signed into law in December.