Updated April 23, 2021 at 4:37 PM ET
The U.S. is joining an international search for a missing Indonesian submarine that lost contact with its base earlier this week. Authorities said the KRI Nanggala 402, if still intact, may by now have exhausted its oxygen supply for its crew of 53.
Pentagon spokesperson John Kirby announced Thursday evening on Twitter that at Indonesia's request, the U.S. was "sending airborne assets to assist in the search for the missing submarine."
The KRI Nanggala 402 disappeared early Wednesday morning while on a training exercise in the Bali Sea after it was given permission to dive. The German-built, diesel-powered boat missed its next radio check-in and has not been heard from since.
An Indonesian military spokesman said officials would push the search hard in the hours before the navy has calculated that oxygen aboard the sub will run out – estimated at about 3 a.m. local time Saturday (4 p.m. ET Friday).
"We will maximize the effort today, until the time limit tomorrow 3 a.m.," Maj. Gen. Achmad Riad told reporters.
Numerous Indonesian navy and other ships and a patrol plane were involved in Friday's last-ditch search, focusing on an area where an oil slick was found earlier in the week, although officials have stressed there's no conclusive evidence it came from the submarine.
India on Thursday dispatched a deep submergence rescue vessel to help in the search, joining Singapore, Malaysia, Australia and other countries that have promised to assist in the search and any possible rescue.
But with the clock ticking, the help is likely to arrive too late.
Indonesian military officials are holding out hope that a rescue is still possible, but they expressed concern shortly after the submarine went missing that it may have gone too deep to recover. Officials have indicated that the KRI Nanggala 402 has a maximum operating depth in the range of 200 to 250 meters (about 655 to 820 feet). But officials said it may have descended much deeper, as far down as 2,300 feet.
In recent days, however, searchers said they discovered an unidentified object with high magnetism at 50 to 100 meters (about 165 to 330 feet) below the surface in the vicinity where they believe the submarine disappeared.
The KRI Nanggala 402 was built in the late 1970s and has been in service in the Indonesian navy since the early '80s. Despite its age, the submarine went through a refit in 2012 in South Korea, and the Indonesian navy insists it had been certified as seaworthy.
The cause of the sub's disappearance is still not certain. The vessel lost contact as it was conducting a weapons exercise, reportedly a torpedo drill, but the navy said an electric failure may have left the sub helpless to execute emergency procedures to resurface after a dive.
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