The security threat unsettled Americans just days before they vote in midterm congressional elections that have been dominated by economic woes rather than the issue of terrorism.
The United States searched on Saturday for the culprits behind a plot to bomb Jewish targets in Chicago uncovered by the interception in Britain and Dubai of parcels with explosives sent from Yemen.
U.S. President Barack Obama vowed on Friday that U.S. authorities would spare no effort to find the source of the packages, which he called a "credible terrorist threat" aimed at two places of Jewish worship.
Obama said security would be increased for air travel for as long as necessary. U.S. officials said they were searching for more packages that could have come from Yemen, which has become a haven for some anti-American militants.
"Initial examinations of those packages have determined that they do apparently contain explosive material," Obama said at a press briefing at the White House.
A parcel intercepted in Dubai and bound for the United States contained a bomb hidden in a printer and bore the hallmarks of al Qaeda, Dubai police said on Saturday.
The parcel contained explosive pentaerythritol trinitrate (PETN) in a printer and cartridge, police said. PETN is the material used in a failed plot to bomb an airline over the U.S. in December 2009.
"The parcel was prepared in a professional way where a closed electrical circuit was connected to a mobile phone SIM card hidden inside the printer," the statement said.
"This tactic carries the hallmarks of methods used previously by terrorist organisations such as al-Qaeda."
Same chemical used in underwear bomb
The New York Times reported that the packages contained PETN, the same chemical explosive used in the bomb sewn into the underwear of the Nigerian man who attempted to blow up an airliner over Detroit at Christmas, a plot hatched in Yemen.
The newspaper cited Representative Jane Harman, a Democrat on the House Homeland Security Committee, who was briefed by the Transportation Security Administration.
The improvised devices contained computer printer cartridges filled with the explosive, but one used a cell phone as a detonator and the other had a timer, she told the Times.
U.S. law enforcement officials would not confirm the report, saying tests were still being done on the explosives.
The White House said "both of these packages originated from Yemen" and Obama was informed of the threat on Thursday.
One of the packages was found on a United Parcel Service cargo plane at East Midlands Airport, about 160 miles (260 km) north of London. The other was discovered at a FedEx Corp facility in Dubai.
UPS and FedEx, the world’s largest cargo airline, said they were halting shipments from Yemen. UPS planes were searched and then cleared in New Jersey and Philadelphia.
One U.S. official and some analysts speculated that the parcels may have been a test of cargo screening procedures and the reaction of security officials.
"This may be a trial run," the U.S. official said.
The white house also said on Friday that Saudi Arabia helped identify the security threat from Yemen.
"The United States is grateful to the Kingdom of Saudi Arabia for their assistance in developing information that helped underscore the imminence of the threat emanating from Yemen," said John Brennan, President Obama’s adviser on counter terrorism.
Brennan said in a statement that the Saudis, as well as officials in Britain, the United Arab Emirates and "other friends and partners," had assisted the United States in identifying the suspicious packages in Dubai and at Britain’s East Midlands Airport.
But Brennan would not say how the United States learned of the plot. But he told reporters: "We were onto this. We were looking for packages that were of concern."
Of the plotters, Brennan said: "Clearly they are looking to identify vulnerabilities in our system. We’ve been able to stay ahead of them."
BA plane met by authorities in NY
U.S. authorities met a British Airways jet from London at New York as a "precautionary measure" the airline said Saturday.
A BA spokeswoman confirmed that U.S. officials had met the flight from London to John F. Kennedy airport and said that all passengers on board had disembarked safely.
"We understand that authorities are meeting the aircraft purely as precautionary measure given the current heightened security situation," the spokeswoman told AFP.
"We are cooperating fully with the authorities."
The spokeswoman did not comment on reports that the cargo of the plane was being searched.
Obama earlier ordered cargo planes at Philadelphia and Newark international airport’s to be towed to isolation and checked because they were thought to contain further packages from Yemen.
U.S. and Canadian fighter jets were scrambled to accompany an Emirates plane into New York but Emirati authorities later said it was not carrying cargo from Yemen.
Yemen launches probe into US-bound parcels
Security and civil aviation authorities "have begun an investigation" into the suspicious packages," the spokesman said on Saturday.
"This probe is being carried out in coordination with the competent authorities in the United Arab Emirates, Britain and the United States, and its results will be announced in due time," he said.
Saba news agency also quoted the unnamed official as saying Yemen would pursue its efforts to "fight against terrorism," amid renewed pressure from the West to stop al-Qaeda militants who are believed to have sent the explosives.