Media playback is unsupported on your device Media caption The Delta-4 Heavy rocket blasted off from Cape Canaveral Air Force station

The US National Reconnaissance Office has launched what is reputed to be the largest satellite ever sent into space.

The spacecraft was put into orbit on a Delta-4 Heavy rocket from Cape Canaveral Air Force station on Sunday.

The NRO gave no details about the payload but it is understood the satellite will be used to eavesdrop on enemy communications.

For comparison, the largest commercial satellite ever launched was the seven-tonne Terrestar-1 telecoms spacecraft.

It had an 18m antenna-reflector to relay phone and data traffic.

US websites have speculated that the mesh antenna on the new NROL-32 satellite would exceed this, and could even be substantially bigger than the 22m-diameter structure orbited last week on another commercial platform called Skyterra-1.

The Delta-4 Heavy rocket, the largest unmanned American launch vehicle, lifted off at 1758 local time (2258 GMT).

It is only the fourth time the giant booster has flown since its maiden outing in 2004.

The rocket features three core boosters strapped side by side. Each has a Rocketdyne-built RS-68 engine, which burns a tonne of propellant every second and produces 2,900 kiloNewtons (650,000lbs force) of thrust at lift-off.

The Apollo Moon rockets, by comparison, could produce more than three times the thrust of the Delta.

The Delta-4 heavy can put up to 13 tonnes in a geostationary transfer orbit (GTO). It does not operate in the commercial market.