But he said that Syria's "continuing support for terrorist organisations and pursuit of weapons of mass destruction and missile programmes, continue to pose an unusual and extraordinary threat to the national security, foreign policy, and economy of the United States".

There was little expectation Obama would lift the sanctions, which he also renewed last year.

In his statement, Obama demanded that Syria demonstrate "progress" before the sanctions could be lifted.

The sanctions - first imposed in 2004 by George Bush, the then president - restrict most US exports to Syria.

Internet curbs remain

US sanctions also prevent Syrians from accessing a number of websites hosted in the US.

SourceForge, a repository for open-source software, blocks Syrian users.

Google does not allow people in Syria to download its Chrome browser.

The social networking site LinkedIn temporarily banned Syrian users last year, though it has since changed its policy.

Activists have urged the US government to lift its digital sanctions, which also apply to several other countries, including Sudan and Iran.

But Washington has not moved to ease those restrictions - even after Hillary Clinton, the US secretary of state, delivered a speech earlier this yearcalling for greater internet freedom.

Obama's announcement of renewed sanctions comes at a time of heightened tensions between the US and Syria.

Scud controversy

The US and Israeli governments both recently accused Syria of equipping Hezbollah with sophisticated Scud missiles. The Syrian government has denied those reports; Hezbollah leaders refuse to comment.

Major-General Alberto Asarta Cuevas, the head of the United Nations peacekeeping force in Lebanon, said last week that he has seen no evidence of Scud missiles in Lebanon.

Obama took office pledging better engagement with Syria, and in February he named Robert Stephen Ford as the US ambassador to Syria.

Ford would be the first American ambassador in Damascus since 2005, when Bush withdrew the US diplomat after the assassination of Rafiq al-Hariri, a former Lebanese prime minister.

But the US senate has yet to confirm Ford's nomination, and it is unclear when legislators plan to vote.