By David Beasley

ATLANTA (Reuters) - Conjoined twin boys, sharing a heart, torso, arms and legs, were born early on Thursday in an Atlanta hospital, marking a medical rarity as many such babies do not survive delivery.

Relatives cheered their arrival at Northside Hospital, where the twins were delivered at 7:32 a.m. local time (EST), according to a hospital spokeswoman.

"Babies are out and so far they are doing AMAZING. Both babies crying their little eyes out!!!" Emily Berdeaux, a relative, wrote on a family Facebook page early on Thursday.

The twins, who share a heart and circulatory system, will not be separated, according to social media posts by their parents, Michael and Robin Hamby of Alabama.

"I need a lot of prayer for my boys," said Michael Hamby on a Facebook video posted on Wednesday night.

Conjoined twins occur once every 200,000 live births and most do not survive, according to the University of Maryland Medical Center. About 40 to 60 percent are stillborn, and about 35 percent live only one day.

The newborn brothers were given medication for their joint heart and intubated to help them breathe, Michael Hamby told the Columbus Ledger-Enquirer newspaper on Thursday morning.

(Editing by Letitia Stein and Gunna Dickson)