An Indian air force plane carrying 29 people went missing on Friday on a flight to a remote island chain in the Bay of Bengal, the Defence Ministry said.
The Russian-made AN-32 was on its way to Port Blair, the capital of India's Andaman and Nicobar islands, from the southeast city of Chennai when it disappeared from radar.
Military officials said the weather had been rough in the Bay of Bengal for the last two days during the monsoon season.
The aircraft was last detected on radar 151 nautical miles east of Chennai, making a left turn with a rapid loss of height from 23,000 ft, according to a note provided by the air force to the Defence Ministry.
There were 21 military personnel on board including six crew. The other people on board were civilians, some family members of soldiers deployed on the islands.
"It was a routine courier mission to Port Blair, the plane was airborne at 8:30 a.m. and due to land at 11:30," air force spokesman Wing Commander Anupam Banerjee said. It had fuel to fly for four hours and 15 minutes.
The plane was overhauled and went through an upgrade in September 2015, according to the note given to the ministry.
The plane had reported three snags this month, according to the note - a pressure leak from the port door, a hydraulic leak and sluggish throttle movement.
The Defence Ministry said four surveillance planes, 12 ships and a submarine were searching for the aircraft in one of India's largest search and rescue operations in recent years.
India has been beefing up its military presence in the Andamans, 750 nautical miles from mainland India, in recent years.
The islands are near the Malacca Straits, one of the world's busiest shipping lanes which link the Indian Ocean to the South China Sea and East Asia.
The Defence Ministry said the submarine had been deployed to locate transmissions from an emergency locator beacon on the aircraft. The AN-32 is a workhorse of the air force, chosen for its ability to operate from short runways.
The air force has 101 of the AN-32s that entered service in 1984 and have gone through mid-life upgrades and life extensions since then.