A ball of fire in the sky over New Brunswick and Nova Scotia that alarmed observers Monday night was caused by rocket debris re-entering the Earth's atmosphere, officials say.

"You know when you see an atomic bomb go off or something? You get that bright intense flash for a second? Well it was kinda like that but solid for a bit," said Robert Jones, a fishermen who spotted the bright light low on the horizon in the Bay of Fundy.

Amateur astronomer Bruno Comeau captured this image in the sky Monday night.

An Atlas V rocket carrying a spy satellite for the U.S. National Reconnaissance Office was launched from Cape Canaveral, Fla., at 5:05 p.m. ET Monday. The largest pieces of debris were seen falling into the Atlantic Ocean off Sable Island about 10 minutes later.

People from all over Charlotte County, in New Brunswick's southwest, Nova Scotia and even the state of Maine called the RCMP to report sighting a ball of flames, said Sgt. Derek Strong.

"We were quite alarmed at first," Strong said.

RCMP in the town of St. George sent two cars rushing to a coastal village to investigate.

"It caught us off guard of course," said Sgt. Greg McEvoy. "First blush, worst-case scenario, we're thinking perhaps a commercial airliner falling from the sky."

Strong said the RCMP called the Search and Rescue Co-ordination Centre in Halifax, which tracks aircraft.

"They were able to determine that in fact it was not an aircraft that was lost but a satellite [rocket] instead, so we were quite relieved," he said.

NASA told rescue centre

Officials at the rescue co-ordination centre said they had been told by NASA to expect rocket debris to fall in the area.

"They would have simply seen the booster stage of the rocket burning up in the atmosphere in re-entry," said 2nd Lt. Noel Paine of the Joint Task Force Atlantic.

The pieces of the rocket splashed into the ocean about 250 nautical miles (400 kilometres) off the coast of Nova Scotia, Paine said.

The rocket, sent up by United Launch Alliance, reached space successfully, said Jim Sponnick, vice-president of Atlas programs in a release.

The classified payload carried by the rocket will ensure that the United States has the technology needed to acquire intelligence data worldwide, Sponnick said.