A suspected bomb was found on Friday in Britain on board a cargo plane headed to the United States, where authorities were investigating other cargo flights for "potentially suspicious items" in New York, Philadelphia and other cities.
The plane, a United Parcel Service flight that stopped in Britain while traveling to Chicago from Yemen, was carrying an ink toner cartridge converted into a bomb, CNN reported.
An FBI source told Reuters that initial tests in Britain revealed no explosives.
British police said the plane carrying the suspicious package was being checked at a distribution center at East Midlands Airport, some 160 miles (260 km) north of London.
The United States has stepped up its training, intelligence and military aid to Yemen after a failed plot to blow up a US passenger plane on Christmas Day 2009, for which the Yemeni wing of al Qaeda claimed responsibility.
UPS said two of its other planes were being checked in Philadelphia, as well as another that landed in Newark, New Jersey. Local media reported cargo planes also had been stopped for investigation in Portland, Maine, and at New York's John F Kennedy International Airport.
"Out of an abundance of caution the planes were moved to a remote location where they are being met by law enforcement officials and swept," the US Transportation Security Administration said.
A UPS spokeswoman said she did not know where the flights had originated. CNN said the UPS plane in Newark had arrived from East Midlands and said flights to the United States from Yemen were being investigated as a precaution.
UPS could not confirm reports of investigations in Maine or at JFK airport in New York.
Also, a UPS truck in New York City was checked for a suspicious package and then cleared, police said.
New York police spokesman Paul Browne declined to comment on whether there were links with the investigations at Philadelphia and Newark airports.
The accused Christmas Day bomber, Umar Farouk Abdulmutallab, has told U.S. investigators he received the explosive device and training from al Qaeda militants in Yemen.
Yemen has been trying to quell a resurgent branch of al Qaeda, which has stepped up attacks on Western and government targets in the Arabian Peninsula country.