President Bush imposed economic sanctions on Syria yesterday in an order that accused Damascus of supporting terrorism, pursuing weapons of mass destruction and failing to stop anti-US guerrillas from entering Iraq.

In the widely expected move, Mr Bush labelled Syria "an unusual and extraordinary threat" and imposed a series of sanctions including a ban on US exports to Syria other than food and medicine.

The action is seen as symbolic because Washington has an arms-length economic and political relationship with Damascus.

The sanctions also sever banking relations with the Commercial Bank of Syria, freeze assets of Syrians and Syrian entities suspected of involvement terrorism or WMD development, and prohibit Syrian flights to and from the United States.

The sanctions were chosen from the Syria Accountability and Lebanese Sovereignty Restoration Act, which the President signed into law last December.

The White House said Bush would consider additional sanctions unless Damascus took steps to end support for Islamic militant groups including Hamas and Hezbollah, withdraw its troops from Lebanon, cease WMD development and co-operate fully with US-led efforts to stabilise Iraq.

"The Syrian Government must understand that its conduct alone will determine the duration of the sanctions," Mr Bush said in a statement that promised a positive US response to constructive actions by Damascus.

Not among the sanctions was an expected curb on future investments by US energy companies, which constitute the only major American corporate presence in Syria. A specific ban on US businesses investing or operating in Syria had been among a menu of sanctions covered by the 2003 Act.

A State Department official said US energy companies that trade and refine Syrian oil would find it difficult to maintain operations there because of the export and banking restrictions.

Mohammad Naji al-Otari, the Syrian Prime Minister, described the measure as "unjust and unwarranted" and told reporters the measure "will not have any impact on us in Syria."