A US military blimp has broken loose from the cable that connected it to the ground and is now freely floating in the air, American military officials say.

The US military blimp, that had the East Coast under surveillance, went loose from its mooring in Aberdeen Proving Ground in Maryland, on Wednesday morning, according to a spokeswoman for the Army installation.

The 243-foot-long, helium-filled blimp is now floating over Pennsylvania and two F-16 fighter jets are flown to track it, North American Aerospace Defense Command (NORAD) spokesman Michael Kucharek said.

The military is working with the Federal Aviation Administration (FAA) to ensure air traffic safety, he added.

The Joint Land Attack Cruise Missile Defense Elevated Netted Sensor System (JLENS) is meant to provide warnings in case of attacks on US soil by cruise missiles, drones or other low-flying weapons.

The Pentagon has reportedly spent $2.7 billion to develop the system.