WASHINGTON - US PRESIDENT Barack Obama said on Friday that two packages from Yemen addressed to Jewish synagogues in Chicago contained explosive material and were a 'credible terrorist threat'. The discovery of the suspicious packages overnight on cargo planes in transit for the United States - one in Dubai and the other in Britain - sparked an international security alert.

Informed late on Thursday, Mr Obama immediately ordered cargo planes at Philadelphia and Newark international airports to be towed to isolation and checked because they were thought to contain further packages from Yemen.

The Pentagon later scrambled fighter jets to intercept a passenger plane, reportedly Emirates Airlines flight 201, which was escorted in to land at New York's John F. Kennedy airport because it had another suspect package.

'We will continue to pursue additional protective measures as long as it takes to ensure the safety and security of our citizens,' Mr Obama told a special press conference at the White House.

The president made it clear he suspected Al-Qaeda's Yemeni-based affiliate of being behind the plot, which could have severe ramifications for the global cargo industry.

'Although we are still pursuing all of the facts, we do know that the packages originated in Yemen,' Mr Obama said. 'We know that Al-Qaeda in the Arabian Peninsula, the terrorist group based in Yemen, continues to plan attacks against our homeland, our citizens, and our friends and allies.'

Packages contain explosives