US authorities are searching for six people after a medical plane bound for Boston, Massachusetts, went missing early Saturday morning.

The Federal Aviation Administration told the BBC that it lost contact with the Gulfstream G-100 and issued a search-and-rescue notice just after 1:00 local time (6:00 GMT).

An air and water search is underway off the coast of Nantucket, a Massachusetts island, according to the US Coast Guard's Northeast District.

The plane departed from Bermuda's L.F. Wade International Airport at 23:40 local time, according to FlightAware data, and was scheduled to land at Boston's Logan International Airport at 1:30 EDT (6:30 GMT).

According to the tail number released by the Coast Guard, the plane belonged to Latitude Air Ambulances, a company that provides international medical flights for patients returning home.

Among those on board were two pilots and two medical staff, both Canadian, Clarence Togeretz, director of flight operations at Latitude, told CBC News.

The crew flew to Bermuda to collect a patient and their relative, who are not Canadian, Togeretz said.

FlightAware's tracker shows 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 1:00 EDT to prepare to meet a plane with an "electrical issue".

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