The U.S.-made Black Hawk helicopter went down in poor visibility, raising to 14 the expected casualties from two air accidents in three days in western Phetchaburi province, army officials said.
The two helicopters had flown over a rugged, rain-soaked region that was hard for rescue teams to reach on foot. On board the Black Hawk were a television cameraman and eight army officers.
A Vietnam War era helicopter that crashed on Sunday killing five people had been despatched to the area to find government officials lost while working in the thick jungle.
Among officers presumed killed on Tuesday were Major General Tawan Ruengsri, a regimental commander of forces west of Bangkok.
"Two groups of people in the area heard the Black Hawk's explosion, but the fate of those on board is yet to be determined," said army spokesman Sansern Kaewkamnerd.
Military helicopter accidents are not uncommon in Thailand, with three Bell UH-1H helicopters crashing in the space of four months from June-October 2008, due to engine trouble or bad weather.