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 packages from Yemen on US-bound cargo planes.
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.
US 'remains vigilant'
UK Home Secretary Theresa May said experts were trying to establish whether the package found in Britain was "a viable explosive device".
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."
Witnesses in the Yemeni capital Sanaa say a security crackdown has begun there, reports BBC Middle East correspondent Jon Leyne in Cairo.
Security forces are searching vehicles and carrying out identity checks, he says.
Saudi tip-off
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 attempted Christmas Day attack.
