A former US Marine who sensationally claimed that his unit stumbled across a secret military rendezvous with an alien spacecraft says he has been threatened.

He even fears that one day he could be killed by sinister forces that are trying to cover up the Pentagon's UFO secrets.

Michael Herrera was part of a US Navy humanitarian mission during the 2009 Sumatra earthquake.

After he was "ambushed" by a paramilitary unit he was searched and had his camera confiscated by the mysterious "soldiers in black".

He was warned several times not to tell anyone about the strange "flying saucer" type craft he had seen in the Sumatran jungle.

After remaining silent about his strange encounter for many years, he told his story last month at a briefing given by Dr Steven Greer's Disclosure Project.

He has now told podcaster Shawn Ryan that the intimidation from shadowy forces within the US military has increased.

"I've had numerous encounters with f***ing helicopters hovering over my house… they were hovering over my dad's house too," Michael said.

"They get so low, where they rattle the walls [and] scare the sh*t out of the animals that we've got.

"[My dad's] dogs were going f***ing crazy at that point and he was concerned because he was home and he was like I've never heard anything like that."

Michael said he's shot video footage of the unmarked black helicopters, and he knows that other UFO whistleblowers – even people within the US government – have been subjected to the same kind of intimidation tactics.