Two suspicious packages found on US-bound planes did contain explosives, President Obama has confirmed.
The suspicious device found at East Midlands Airport
The latest development comes amid a huge terror alert on both sides of the Atlantic, which included US fighter jets escorting a passenger plane into New York.
Mr Obama said it was a "credible" terror threat and he had told officials to take "whatever measures were necessary" to protect the US.
"Although we are still pursuing all the facts, we do know that the packages originated in Yemen," he added.
"We also know that al Qaeda in the Arabian Peninsula (Aqap), a terrorist group based in Yemen, continues to plan attacks against our homeland, our citizens, and our friends and allies."
The first explosive was discovered on a United Parcel Service jet, which had stopped to refuel at East Midlands Airport in the UK while flying from Yemen to Chicago.
It was a converted ink toner cartridge, containing white powder, and had wires sticking out from the top.
The British Government has now stopped all flights landing in the UK from Yemen.
The second explosive was found on a plane in Dubai, which had also flown from Yemen and was on its way to the US.
The American government is investigating whether the packages were a dry run for a mail bomb plot on US soil.
The packages were addressed to two synagogues in Chicago.
The Anti-Defamation League (ADL) had warned Jewish communities in the US about packages sent from overseas - particularly from Britain, Yemen and Saudi Arabia.
Earlier, fighter jets escorted an Emirates flight from Dubai into John F Kennedy Airport.
FBI spokesman Richard Kolko said the action was taken because the Boeing 777 was carrying cargo from Yemen.
Sky sources said security officials are checking to see whether any other planes are carrying packages from Yemen.
Sky's crime correspondent Martin Brunt said the UK package was found on the cargo plane at about 3.30am this morning.
“The package was checked and at some stage was given the all-clear because the plane was allowed to continue its flight to North America," he added.
“For some reason this package was revisited and then concern grew again - so much so that for the last 12 hours or so this police operation has continued."
Mr Obama said he was told of a "potential terrorist threat" from Yemen late on Thursday night.
After the alert, two UPS planes were searched at Philadelphia International Airport. One had flown in from Paris, France, the other from Cologne, Germany.
Another UPS plane was searched at Newark Liberty International Airport in New Jersey. It had taken off from East Midlands Airport.
Two vehicles in New York City were also investigated, as well as a backpack found in a shrubbery outside a court in Portland, Maine.
Terrorism expert Neil Livingstone told Fox News: “Clearly they probably had some intelligence that indicated there were going to be bombs placed on cargo planes and they’ve been able to mobilise very quickly to deal with likely flights.
“The cargo hold has always been of great concern in cargo planes because we’ve never really got our arms around the security of those planes.”
But aviation security expert Chris Yates warned: “I think in some ways we need to wind our necks in and be circumspect about everything that’s happening at the moment.
“The reports that I’m hearing suggest that yes, there is a concern out there… that some threat items may have been found - maybe not.
“So we just have to wait and see as this story plays out as to whether there is a serious and real threat to the cargo sector and cargo planes flying between key points around the world.”
The suspicious device found at East Midlands Airport
The latest development comes amid a huge terror alert on both sides of the Atlantic, which included US fighter jets escorting a passenger plane into New York.
Mr Obama said it was a "credible" terror threat and he had told officials to take "whatever measures were necessary" to protect the US.
"Although we are still pursuing all the facts, we do know that the packages originated in Yemen," he added.
"We also know that al Qaeda in the Arabian Peninsula (Aqap), a terrorist group based in Yemen, continues to plan attacks against our homeland, our citizens, and our friends and allies."
The first explosive was discovered on a United Parcel Service jet, which had stopped to refuel at East Midlands Airport in the UK while flying from Yemen to Chicago.
It was a converted ink toner cartridge, containing white powder, and had wires sticking out from the top.
The British Government has now stopped all flights landing in the UK from Yemen.
The second explosive was found on a plane in Dubai, which had also flown from Yemen and was on its way to the US.
The American government is investigating whether the packages were a dry run for a mail bomb plot on US soil.
The packages were addressed to two synagogues in Chicago.
The Anti-Defamation League (ADL) had warned Jewish communities in the US about packages sent from overseas - particularly from Britain, Yemen and Saudi Arabia.
Earlier, fighter jets escorted an Emirates flight from Dubai into John F Kennedy Airport.
FBI spokesman Richard Kolko said the action was taken because the Boeing 777 was carrying cargo from Yemen.
Sky sources said security officials are checking to see whether any other planes are carrying packages from Yemen.
Sky's crime correspondent Martin Brunt said the UK package was found on the cargo plane at about 3.30am this morning.
“The package was checked and at some stage was given the all-clear because the plane was allowed to continue its flight to North America," he added.
“For some reason this package was revisited and then concern grew again - so much so that for the last 12 hours or so this police operation has continued."
Mr Obama said he was told of a "potential terrorist threat" from Yemen late on Thursday night.
After the alert, two UPS planes were searched at Philadelphia International Airport. One had flown in from Paris, France, the other from Cologne, Germany.
Another UPS plane was searched at Newark Liberty International Airport in New Jersey. It had taken off from East Midlands Airport.
Two vehicles in New York City were also investigated, as well as a backpack found in a shrubbery outside a court in Portland, Maine.
Terrorism expert Neil Livingstone told Fox News: “Clearly they probably had some intelligence that indicated there were going to be bombs placed on cargo planes and they’ve been able to mobilise very quickly to deal with likely flights.
“The cargo hold has always been of great concern in cargo planes because we’ve never really got our arms around the security of those planes.”
But aviation security expert Chris Yates warned: “I think in some ways we need to wind our necks in and be circumspect about everything that’s happening at the moment.
“The reports that I’m hearing suggest that yes, there is a concern out there… that some threat items may have been found - maybe not.
“So we just have to wait and see as this story plays out as to whether there is a serious and real threat to the cargo sector and cargo planes flying between key points around the world.”