Barack Obama warned that the terror threat posed by packages found on cargo flights in Britain and Dubai was "credible" tonight amid a major security alert in US cities.
The President said the packages, which originated in Yemen, had both contained explosive material and the targets appeared to be two synagogues in Chicago.
Mr Obama said he would be seeking greater "co-operation" with the state in order to tackle the activities of an al Qaida offshoot, and urged the public to be "vigilant".
Speaking in the White House, Mr Obama said: "I want to briefly update the American people on a credible terrorist threat against our country and the action that we have taken with our partners to respond to it.
"Last night and earlier today our intelligence and law enforcement professionals working with our friends and allies identified two suspicious packages bound for the US - specifically, two places of Jewish worship in Chicago.
"Those packages have been located in Dubai and East Midlands Airport in the UK.
"Initial examination of those packages has determined that they do apparently contain explosive material."
The president's comment seemingly contradicts earlier suggestions that the package found in Britain did not contain explosives.
Mr Obama said he had instructed officials to take "whatever measures were necessary" to protect Americans, and planes in Newark and Philadelphia had been searched as part of tougher cargo screening.
"We will continue to pursue additional protective measures for as long as it takes to ensure the safety and security of our citizens," he added.
"I have also directed that we spare no effort in investigating the origins of these suspicious packages and their connection to any additional terrorist plotting.
"Although we are still pursuing all the facts, we do know that the packages originated in Yemen. We also know that al Qaida in the Arabian Peninsula, a terrorist group based in Yemen, continues to plan attacks against our homeland, our citizens, and our friends and allies."
Mr Obama said Yemen president Ali Abdullah Saleh had pledged "full co-operation" to hunt down the terrorists.
"Going forward we will continue to strengthen co-operation with the Yemen government to disrupt plotting by al Qaida in the Arabian Peninsula, and to destroy this al Qaida affiliate.
"We will also continue our efforts to strengthen a stable, secure and prosperous Yemen so the terrorist groups do not have the time and space they need to plan attacks from within its borders."
Mr Obama added: "The events of the last 24 hours underscores the necessity of remaining vigilant against terrorism."
Police were called to a freight distribution centre at East Midlands Airport early this morning after officials raised concerns during a routine search about a flight that originated in Yemen.
Workers are believed to have found a toner cartridge which had been altered with wires attached to it, and a white powder.
Soon afterwards, a suspect device was found in Dubai.
Officials moved two United Parcel Service jets in Philadelphia and a third jet in Newark, New Jersey, away from terminal buildings.
In Brooklyn, New York, police were examining a package from a UPS truck after reports that a possible explosive had been found. An Emirates flight from Dubai was escorted safely into New York by fighter jets.
Officers from Scotland Yard's Counter Terrorism Command have been liaising with colleagues across the Atlantic.
Home Secretary Theresa May said: "At this stage there is nothing to suggest that any location in the UK was being targeted.
"We are urgently considering what steps need to be put in place regarding security of freight originating from Yemen.
"For security reasons there are currently no direct flights from Yemen to the UK."
Concerns have been growing about the threat posed by Yemen-based terrorists for some time.
Direct flights from the country to the UK were halted in January after a plot to bomb a plane bound for Detroit was foiled.
In April, an al Qaida suicide bomber almost succeeded in assassinating Tim Torlott, Britain's ambassador to Yemen.
And earlier this month, terrorists targeted Fiona Gibb, the deputy chief of the UK's embassy in the capital Sana'a.Reuse content