Two members of the Kennedy political dynasty, including a grandniece of John F Kennedy, are presumed dead after they went missing during a canoe trip.

It looks to be the latest chapter of heartbreak for a family that has suffered numerous tragedies in the form of untimely deaths, since President Kennedy himself was assassinated in Dallas in 1963.

This time it involves a grandniece of the late president, Maeve Kennedy McKean, 40, and her eight-year-old son Gideon. 

They disappeared on Thursday while canoeing in the South River in Maryland, near the vast Chesapeake Bay, Governor Larry Hogan said.

The coast guard, police and firefighters immediately launched a search but did not find the missing Kennedys.

David McKean with his wife Maeve Kennedy McKean and family 

"It has been more than 24 hours, and the chances they have survived are impossibly small. It is clear that Maeve and Gideon have passed away," husband David McKean wrote on Facebook.

The rescue operation has now shifted to one of finding the bodies, said Kathleen Kennedy Townsend, the mother and grandmother of the missing two.

Kennedy Townsend, 68, is a former Maryland lieutenant governor, and the daughter of Robert F Kennedy, the one-time US attorney general in his brother's administration.

RFK was himself slain in 1968 as he campaigned for president, five years after his brother's death.

In 1999 John F Kennedy Jr and his wife and sister-in-law died when the small plane he was piloting crashed off the Massachusetts coast.

Saoirse Kennedy Hill, a granddaughter of RFK, died of what was ruled an accidental drug overdose last year at age 22.

David Kennedy, a son of Robert, died at age 28 of a cocaine overdose at a Florida hotel in 1984. 

Another son of RFK, Michael, died in a skiing accident in Colorado in 1997.