All nine people on board an aging US cargo plane have been killed when the aircraft making its final military flight nose-dived into a highway in the US state of Georgia, an official said.

They were aboard the C-130 plane when it came down on Wednesday beside the highway intersection near the airport in the coastal city of Savannah.

The plane belonged to Puerto Rico Air National Guard and was on a training flight, the National Guard said. All the airmen were from Puerto Rico, an official said.

Images shared on social media showed wreckage engulfed in flames and black smoke spewing into the sky, the BBC reported.

It was flying from Georgia to Tucson in Arizona, where it was to be decommissioned. The plane was at least 50 years old, according to an official familiar with the aircraft.

"Nine crew members died in the accident, but until their families and relatives are notified, we cannot give their names," Brig. Gen. Isabelo Rivera said. "Our prayers, thoughts and condolences to the families and loved ones of our aviators."

One witness said the plane made a "loud, strange noise" just before the crash. While another said the ground shook as if a bomb went off when it hit.

Video from a business near the crash showed the horrifying final moments of the hulking plane, a version normally outfitted to do weather reconnaissance, the CNN report said.

The jet, with four turboprop engines on its overhead wing, banked left as it came down. The plane then headed straight down behind trees. Seconds later a fireball and thick black smoke appeared.

Puerto Rico's Governor sent his condolences. "While we are waiting for more information regarding this unfortunate accident, my thoughts and those of Beatriz are with the families of the crew..., " Governor Ricardo Rossello said.

President Donald Trump tweeted: "Please join me in thoughts and prayers for the victims, their families and the great men and women of the National Guard."