Passengers disembark from Emirates Airlines Flight 201 after the plane was escorted by U.S. fighter jets to John F. Kennedy International Airport in New York on Friday afternoon. (CBC)

U.S. President Barack Obama said Friday that suspicious packages found on planes in the United Kingdom and Dubai contained explosive materials that were bound for the U.S.

Obama said the packages were headed for "two places of Jewish worship in Chicago" and that he has directed authorities to take whatever steps necessary to protect Americans from a terrorist attack.

"An initial examination of those packages has determined that they do apparently contain explosive material," he said.

U.S. President Barack Obama tells reporters in Washington, D.C., that suspicious packages found on U.S.-bound planes in the United Kingdom and Dubai contained explosive materials. (Charles Dharapak/Associated Press)

U.S. officials told The Associated Press that they believed the two packages contain the same powerful explosive used in the failed Christmas Day 2009 airline bombing.

The officials said full testing has not been completed, but initial indications are the packages contained PETN, a chemical that was also a component of shoe bomber Richard Reid's explosive in 2001.

Obama was notified of a potential threat Thursday evening after intelligence and law enforcement agencies discovered suspicious packages in airports in Dubai and East Midlands, U.K. The flights had originated in Yemen.

"We've identified these two packages right now. They have been isolated and they have been made inert," White House Homeland Security Adviser John Brennan said at a news conference later Friday. "So therefore what we're trying to do now is ensure we're able to address any other threats that might be out there."

Brennan said other packages are being examined to determine if they contain material of concern, but did not elaborate.

The first package that contained explosive materials was discovered in East Midlands, he said, adding that packages coming from Yemen will be more carefully screened.

The concern led to additional screening of some planes in Newark, N.J., and Philadelphia earlier Friday, and an escort of an Emirates plane to New York City that was thought to be carrying cargo from Yemen as a precaution.

Obama said that the government also knows that al-Qaeda in the Arabian Peninsula, a terrorist group based in Yemen, continues to plan attacks against the U.S.

Yemen is the home of the al-Qaeda branch that claimed responsibility for an attempted bombing of a U.S.-bound jet last Christmas.

"We will continue to pursue additional protective measures for as long as it takes to ensure the safety and security of our citizens," Obama said.

Authorities investigate UPS containers in the cargo area of East Midlands Airport, near Nottingham central England on Thursday. (David Jones/Associated Press)

"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."

Earlier, authorities said a package en route to the U.S. from Yemen containing a toner cartridge, wires and powder was found Thursday evening during routine screening of UPS cargo in the United Kingdom. But the investigation there turned up nothing threatening.

Another suspicious package, also en route to the U.S. from Yemen, was found in Dubai and was being delivered by FedEx. An official United Arab Emirates security source told The Associated Press a device discovered in that air cargo shipment contained explosive materials. But no further details were available.

A U.S. official said the devices in England and Dubai were found after Saudi Arabian intelligence uncovered information related to Yemen and passed it to the U.S.

As part of the ongoing investigation, an Emirates airliner suspected of carrying cargo from Yemen landed safely at the John F. Kennedy International Airport in New York City after being escorted from the Canadian border to the U.S. by military fighter jets.

As part of the North American Aerospace Defence Command (Norad) system, the flight was initially intercepted by two Canadian CF-18s from CFB Bagotville after it entered Canadian airspace early Friday afternoon.

The CF-18s began shadowing the Emirates airliner when it was about 110 kilometres northeast of Bagotville. They followed the plane across northern New Brunswick to Maine. The Canadian jets then handed off to the two U.S. fighters as the aircraft entered U.S. airspace.

Shortly afterwards, investigators determined the flight wasn't carrying any cargo from Yemen.

A spokesman for Transport Minister Chuck Strahl said the government had no information that the incidents were targeting Canada.

"As a result of these incidents, Canada has implemented increased vigilance," said John Babcock, adding that travellers may see signs of heightened scrutiny.

"Our government always considers the balance between highest level of security and passenger comfort," he said.

With files from The Associated Press