The Indian Air Force has lost contact with a transport plane carrying 29 people to islands in the Bay of Bengal, the country's defence ministry said.
The air force, navy and coast guard were using ships, planes and helicopters to conduct a massive search.
The AN-32 aircraft took off from the southern city of Chennai at 8.30am local time and was flying to the city of Port Blair in the Andaman and Nicobar islands.
The flight, covering 854 miles, was supposed to last three hours. The Russian-made aircraft could go for four hours without refuelling, defence ministry spokesman Nitin Wakankar said.
The plane was making a routine courier flight and had on board military personnel along with the aircraft crew.
The last communication from the aircraft was about 15 minutes after take-off, air force spokeman Anupam Banerjee said.
It was not known if weather was bad during the flight, though monsoon season brings heavy winds and storms that can cause turbulence and flight delays around south and south-east Asia.
The defence ministry said four naval ships deployed in the Bay of Bengal on different missions have joined the search and eight other ships are returning from the South China Sea to join the operation.
A submarine has also been diverted to the area to help locate transmissions from the emergency locator beacon on board the aircraft.
