SAN SALVADOR, March 1 (Reuters) - An inmate at an El Salvador jail was caught with a hand grenade stuffed up his backside -- a novel attempt to disguise his apparent escape plans.

Guards at the San Francisco Gotera prison outside the capital San Salvador found the V40 grenade, about the size of a golf ball, lodged up the man's rectum during a security clampdown, a prison spokesman said on Thursday.

They also caught another 16 inmates who each swallowed a mobile phone.

"We'll have to expel the objects and if they won't come out we'll have to perform surgery in hospital," said Alberto Uribe, a spokesman for the El Salvador prison service.

Last year, prison guards found an M67 grenade in the vagina of a female visitor at the overcrowded La Esperanza-Mariona prison on the northern fringes of San Salvador.

Prisoners in the Central American country use weapons to try to escape or attack fellow inmates and prison guards, and use cellular phones to order free gang members to commit crimes or smuggle narcotics.