Flames raged for hours on Sunday aboard a U.S. warship docked at Naval Base San Diego, sending 21 people to the hospital with minor injuries and prompting a relocation of two other Navy vessels moored nearby, military and local fire officials said.

The San Diego Fire-Rescue Department reported an explosion in conjunction with the blaze, which broke out at about 8:30 a.m. local time aboard the USS Bonhomme Richard, an amphibious assault vessel in port for routine maintenance.

The blast was believed to have been triggered by the rapid initial release of heat from a confined space under pressure, as opposed to an explosion of fuel or ordnance, Rear Adm. Philip Sobeck told a news conference on Sunday evening.