Six people aboard a medical transport aircraft that went missing on Saturday while it was travelling from Bermuda to Boston are presumed dead, according to a statement from the flight's operator.

On Sunday evening, the United States Coast Guard said it was ending its search for survivors off the coast of Nantucket, a Massachusetts island.

"We must now face the heartbreaking reality that the four members of our Latitude team, our patient and the patient's family escort, who were aboard Heartbeat 104, are presumed to have died," Latitude Air Ambulance said in a statement shortly thereafter.

"Our thoughts are first and foremost with their families, loved ones, friends, and colleagues."

The Federal Aviation Administration lost contact with the Gulfstream G-100 plane just after 01:00 local time (05:00 GMT) on Saturday morning.

On Saturday evening, authorities located a "debris field" confirmed to be from the aircraft.

The plane departed from Bermuda's L.F. Wade International Airport at 23:40 local time on Friday night, according to FlightAware data, and was due to land at Boston's Logan International Airport at 01:30 EDT (05:30 GMT).

Four Canadian nationals and two Bermudian nationals were onboard.

"This is an unimaginable loss for our Latitude family and for everyone who loved the six people aboard Heartbeat 104," said Peter Byl, President of Latitude Air Ambulance, on Sunday evening.

"Our focus now is on supporting their families, supporting our people and assisting the authorities. We will honour our colleagues by remembering the care, professionalism, and commitment they brought to their work every day."

Latitude Air Ambulance is a Canadian company that provides international medical flights for patients returning home.

Two pilots and two medical staff, both Canadian, had flown to Bermuda to collect a patient and their relative, Clarence Togeretz, director of flight operations at Latitude, told CBC News.

FlightAware's tracker shows that the aircraft significantly dropped in altitude before its communications went dark, plummeting from around 7,300m (24,000ft) to 3,300m (11,000ft).

Radio transmissions show trouble with communications between the air traffic controller and the pilot, the Boston Globe reported, based on transmissions provided by LiveATC.net.

"You're coming in blind? Is that what you said?" the air traffic controller asked. "Are you declaring an emergency, or no?"

The pilot's response was inaudible.

"You're coming in a little weak so I didn't hear what your response was, if you were declaring an emergency or not," the air traffic controller said, according to the Globe.

The air traffic controller later confirmed the pilot had declared an emergency.

An area fire dispatcher asked emergency crews just after 01:00 EDT to prepare to meet a plane with an "electrical issue".

The dispatcher later said "the aircraft has lost radio and radar".

Latitude said in a statement on Sunday evening that it would continue to cooperate with the relevant authorities in ongoing recovery efforts and investigations.

"Determining the circumstances and cause of an aviation accident is a careful and methodical process, and we will not speculate about what occurred," the company said.