Jakarta(CNN) Concerns were mounting Thursday over the fate of an Indonesian submarine that went missing with 53 crew members on board, as a military official warned the vessel would run out of oxygen in three days.
Admiral Yudo Margono, the chief of staff of the Indonesian Navy, said the submarine has sufficient oxygen for 72 hours, based on calculations from when the vessel lost contact during a military exercise on Wednesday.
The German-made KRI Nanggala-402 asked for permission to dive, or submerge, at 3 a.m. local time Wednesday before losing contact, authorities said. Margono said the submarine had just fired two torpedoes -- one with real ammunition and another with a practice warhead -- as part of training exercises in the Bali Strait, a stretch of water between the islands of Java and Bali that connects to the Indian Ocean and Bali Sea.
Answering questions on the submarine's condition before participating in the war simulation, Margono said the KRI Nanggala-402 and all of its crew are well prepared. It last docked for maintenance in 2020 in Surabaya, a port city on the island of Java, he said.
The military suspects that an oil spill seen in aerial surveillance near the dive point on Wednesday came from the craft. Margono said the Navy also found one object at the depth of 50-100 meters (approximately 164-328 feet) that was magnetic, meaning it likely came from the submarine.
Margono said there are two possibilities to explain the oil spill spotted on the surface: the submarine tank could be leaking because it dove too deep, or the submarine released fluid on board in an attempt to rise to the surface.
Indonesian Navy spokesman First Adm. Julius Widjojono said that the submarine has the capability to dive up to 500 meters (approximately 1,640 feet) below sea level, but authorities estimate it went 100-200 meters below that depth.
Two ships equipped with side-scan sonar, a tool used for mapping the seafloor, began searching the area Wednesday, the Ministry of Defense said, while a Rigel warship equipped with sophisticated sonar that can precisely detect the vessel's position is en route from Jakarta, according to Widjojono.
Authorities are holding out hope the crew are safe but acknowledged the situation could be fatal at that depth.
"Let's pray for them, so they can survive," Widjojono told local media Wednesday.
The International Submarine Escape and Rescue Liaison Office (ISMERLO), an organization that facilitates an international response for distressed submarines, is also providing assistance, the Ministry of Defense said.
The 1,395-ton KRI Nanggala-402 was built in 1977 by the German shipbuilding company Howaldtswerke-Deutsche Werft (HDW) and joined the ranks of the Indonesian Navy in 1981, according to a ministry statement.
The submarine underwent a two-year refit in South Korea that was completed in 2012, according to the Indonesian cabinet secretariat.
In the past, Indonesia operated a fleet of 12 submarines purchased from the Soviet Union to patrol the waters of its sprawling archipelago. But now it has a fleet of only five, including two German-built Type 209 submarines and three newer South Korean vessels.
Indonesia has been seeking to upgrade its defense capabilities, but some of its equipment still in service is old and there have been deadly accidents in recent years, in particular involving aging military transport planes.