An Indonesian KRI Nanggala-402 submarine in September 2014 in Surabaya, East Java, Indonesia.

Indonesia’s Navy is searching for a missing submarine with 53 people on board that went missing during a military exercise on Wednesday, Indonesian authorities say.

In a statement published Wednesday night, the Indonesian Ministry of Defense said the KRI Nanggala-402, a German-made submarine, lost contact during a torpedo drill in the Bali Strait – a stretch of water between the islands of Java and Bali that connects to the Indian Ocean and Bali Sea.

Members of National Search and Rescue Agency (BASARNAS) prepare for a search mission for Indonesian Navy submarine KRI Nanggala at Benoa harbor in Bali on Wednesday.

Adm. Yudo Margono, chief of staff of the Indonesian Navy, said at a news conference Thursday that the vessel has sufficient oxygen for all the submariners until Saturday at 3 a.m. local time.

Three submarines, five airplanes and 21 warships have been deployed to help search for the missing sub, Margono said. Assets from Singapore and Malaysia are also en route to assist.

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, Indonesian Navy spokesman First Adm. Julius Widjojono said.

The submarine 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 a training exercises.

Widjojono said the submarine has the capability to dive up to 500 meters below sea level, but authorities estimate it went 100-200 meters below that depth.

The military suspects that an oil spill seen in aerial surveillance near the dive point came from the ship. Margono said the navy also found one object floating in the depth of 50-100 meters that was magnetic, meaning it likely came from the sub.

It’s unclear exactly what caused the oil spill. Margono, the navy chief of staff, said a tank could be leaking because the submarine dove too deep or that the submariners may be releasing all fluids on board to help the submarine float to the surface.

Authorities are holding out hope the crew are safe, but acknowledged the situation could be fatal at that depth, Widjojono said.

“Let’s pray for them so they can survive,” he 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 said.

Indonesian marine police prepare to take part in the search operation.

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 the Ministry of Defense statement.

The submarine underwent a two-year refit in South Korea that was completed in 2012, according to the Indonesian cabinet secretariat’s website.

Indonesia in the past 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.

CNN’s Kara Fox contributed to this report.