Former U.S. president Jimmy Carter says his 28-year-old grandson has died.

Carter revealed the tragic news to his church congregation in Plains, Georgia on Sunday, just two weeks after learning that his own brain cancer had gone into remission.

Jeremy Carter, 28, died in hospital when his heart stopped, the former president said.

Carter said Jeremy wasn't feeling well and decided to take a nap. Jeremy's mother tried to wake him, only to learn his heart had "quit beating." He was rushed to hospital and his heart briefly restarted, but stopped again early in the morning.

"He was just 28 and a very wonderful young man whom we loved very much," Carter told his congregation.

The elder Carter, who is 91, described his grandson as a "very special child."