Two U.S. Marines have died in separate incidents, one of a presumed heart attack while exercising, the other of a suspected brain aneurysm, military officials said Wednesday.

The deaths, which occurred Monday, raised the number of the U.S. deaths in Operation Desert Shield to 56, including 49 in air crashes and other accidents, according to the U.S. Central Command.Names of the two marines - one aboard the USS Iwo Jima and the other from Camp Lejeune, N.C. - were withheld pending notification of relatives.