An Indonesian navy submarine conducting a training exercise with 53 people aboard was reported missing Wednesday.

Hadi Tjahjanto, commander of the Indonesian National Armed Forces, told Reuters that contact had been lost with the vessel early Wednesday, and that warships were searching waters about 60 miles north of Bali, an Indonesian island.

Rescuers found an oil spill while searching for the vessel, the news agency reported, but have not located the submarine itself.

The first admiral of Indonesia's navy, Julius Widjojono, confirmed to Agence France-Presse that the submarine could not be reached.

Reuters reported that Indonesian officials had asked on Australia, India and Singapore for assistance in the search for the KRI Nanggala-402, a German-made vessel.

The submarine, one of five operated by Indonesia's military, was built in the 1970s and refitted in 2012, according to media reports.

Indonesia is the world's fourth largest country by population, and its vast waters, around an archipelago of more than 17,000 islands, span more than 3,000 miles east to west, and more than 1,000 north to south.

The Australian, Indian, Indonesian and Singaporean militaries did not immediately respond to requests for comment.

It is unusual for a naval submarine to disappear.

In 2017, a submarine carrying 44 people disappeared off the coast of Argentina, prompting international search and rescue efforts. The Argentine navy confirmed soon afterward that an "anomalous, singular, short, violent and non-nuclear event, consistent with an explosion, was registered," near where the ARA San Juan had last been located.

The rescue mission was eventually called off, and the wreckage was not found for a year.

In 2000, a Russian submarine carrying 118 people disappeared in the Barents Sea, and Russian officials later said torpedoes had exploded on board. A note recovered from the ship suggested that at least 23 people may have initially survived but were not rescued in time. There were no survivors.

There have been several high-profile sea searches in Indonesia in recent years.

In January, search-and-rescue operations were launched in the Java Sea after a Boeing 757-500 carrying 62 people crashed shortly after takeoff from the capital of Jakarta. There were no survivors.

In 2018, a Boeing 737 Max flown by Indonesian airline Lion Air crashed into the Java Sea shortly after takeoff. All passengers and crew were killed, and the incident came under further scrutiny soon after when another 737 Max, flown by Ethiopia Airlines, crashed and killed everyone aboard. All 737 Max airliners were then grounded for a lengthy investigation.

Four years earlier, an AirAsia Airbus A320 crashed into the Java Sea after taking off from Jakarta.