A US-bound parcel intercepted in Dubai contained explosives and bore al-Qaeda hallmarks, Dubai police said.

The parcel contained pentaerythritol trinitrate (PETN) - the same explosive used in a failed plot to bomb a plane to Detroit in December 2009.

On Friday, security officials in the UK and Dubai intercepted two cargo planes bound for the US from Yemen.

Intelligence agencies believe the packages are linked to Al-Qaeda in the Arabian Peninsula, based in Yemen.

The two suspect packages - described by President Barack Obama as “a credible terrorist threat” - were addressed to Jewish synagogues in the Chicago area.

The packages were found on UPS and FedEx cargo planes, triggering alerts in the US, UK and Middle East. Other planes at US airports were checked because they were thought to contain items from Yemen.

UK Home Secretary Theresa May said experts were trying to establish whether the package found in Britain was “a viable explosive device”.

Yemen has risen rapidly towards the top of the list of countries of concern for Western counter-terrorism officials in the past year. The group al-Qaeda in the Arabian Peninsula - which has found a sanctuary in Yemen’s ungoverned spaces - has shown increasing ambition and sophistication in its attempts to target the United States and others.

But while the origin of this plot seems clear, its exact form does not. The presence of explosives in the devices suggests this was no dry run or simply an attempt to cause panic through a hoax.

But forensic experts in the UK have been continuing to study the substances found in the parts for a printer to try to understand exactly what they were and how they were to be used. There appears to be a strong conviction these were parts for a bomb, but whether they were complete, how they were to be detonated and against which target remains uncertain.

Mr Obama’s top counter-terrorism adviser John Brennan said: “The United States is not assuming that the attacks were disrupted and is remaining vigilant.”

Speaking at a White House press conference late on Friday, President Obama said: “Although we are still pursuing all the facts, we do know that the packages originated in Yemen.

“We also know that al-Qaeda in the Arabian Peninsula - a terrorist group based in Yemen - continues to plan attacks against our homeland, our citizens and our friends and allies.”

He stressed that new aviation security measures were being taken in light of the alert by the Department of Homeland Security, “including additional screening”.

The White House later said Saudi Arabia had provided information that helped identify the threat.

The UK’s Daily Telegraph reported that an MI6 officer responsible for Yemen had received a tip-off.

FedEx and UPS suspended all their shipments out of Yemen, saying they would fully co-operate with investigators.

Speaking in London early on Saturday, Mrs May said that “at this stage, I can say that the device [found in Britain] did contain explosive material”.

“The forensic work continues,” she said, adding that the British government’s emergency committee, known as Cobra, had met on Friday and would hold another meeting later on Saturday.

“We are reviewing the security measures for air freight from Yemen and are in discussion with industry contacts,” she said.

US security services remain on a high level of vigilance in the wake of the attempted Times Square bombing in New York in May and the alleged attempted Christmas Day attack.