US law enforcement officials are today investigating reports of suspicious packages on cargo planes in the eastern US cities of Philadelphia and Newark, New Jersey.

Transportation Security Administration spokeswoman Kristin Lee said the planes landed safely and are being swept by law enforcement.

The planes have been moved to a remote location so they can investigate.

Philadelphia International Airport spokeswoman Victoria Lupica said Philadelphia police and other agencies are with a plane at a remote part of that airport. Video from the airport shows emergency vehicles around a UPS jet.

A spokeswoman for United Parcel Service did not immediately know what prompted authorities to search the flight at Philadelphia. The company says it is working closely with authorities.

An unconfirmed report by CNN said that a bomb had been discovered on a UPS flight to Chicago from Yemen when it was stopped in London.

CNN said the alert in London centered on an ink toner cartridge converted into a bomb.

President Obama has called the incidents a “serious threat.”

In a statement from DHS: "As a precaution, DHS has taken a number of steps to enhance security. Some of these security measures will be visible while others will not. The public may recognize specific enhancements including heightened cargo screening and additional security at airports. Passengers should continue to expect an unpredictable mix of security layers that include explosives trace detection, advanced imaging technology, canine teams and pat downs, among others. As always, we remind the public to remain vigilant and report suspicious activity to local law enforcement."