The US has imposed economic sanctions on Syria after long accusing the Arab state of supporting terror and failing to stop militants entering Iraq.
The gap between Washington and Damascus is widening even further
President George W Bush ordered a freeze on certain Syrian assets in
the US and a halt to all American exports apart from humanitarian items.
He accused Syria of continuing to occupy Lebanon and pursuing weapons of
mass destruction and missiles.
Syria has denied wrongdoing and says sanctions will only harm US interests.
The sanctions were authorised under the Syria Accountability Act, signed into law by President Bush in December.
They include a ban on flights between Syria and the US but exports of US food, medicines and aircraft parts - for safety reasons - are not affected.
The BBC's Jon Leyne reports from Washington that the announcement had been expected but still marks a significant worsening of relations between the two countries.
Washington has effectively decided to isolate Syria in the hope that tough action will finally produce a change in behaviour, our correspondent says.
In his statement, Mr Bush said Syria's actions amounted to an "unusual and extraordinary threat to the national security, foreign policy and economy" of the US.
SYRIA'S FOREIGN TRADE
Bilateral trade with US worth an annual US $0.3 billion
Bilateral trade with EU: $7.2 bn (2002 figures)
Chief exports: petroleum products and textiles
He accused Syria of continuing to harbour Palestinian militants and supporting the Hezbollah group in Lebanon, and of maintaining a military force in Lebanon against the spirit of peace accords there.
Damascus also possessed "one the most advanced Arab state chemical weapons capabilities", the statement said.
On Iraq, the US leader accused President Bashar al-Assad of supplying Saddam Hussein's forces just before the US-led invasion and of since then becoming "a preferred transit point for foreign fighters into Iraq".
Syrian Prime Minister Mohammed Naji al-Otari said the sanctions were "unjust and unjustified".
Ahmed Haj Ali, media adviser to the Syrian information
minister, told AP news agency that the sanctions would have little economic impact but "much bigger" political effects.
However, he added that stalled trade and political negotiations with the EU - a hugely more important trade partner, accounting for some 60% of Syria's exports - could be affected by US pressure.
Raymond Elias, a Syrian businessman who deals in pumps and generators imported from Europe, told AP he thought that many US firms would ignore the sanctions.
"Maybe some... may bend to the [Bush] administration but many others at the end will be looking after their own interests," the businessman said.