A former US army sniper and two other ex-American soldiers agreed to become contract killers for an international crime boss who wanted to settle a score with a real estate agent in the Philippines he thought had cheated him on a land deal, a prosecutor has told a US federal court.

Joseph Hunter, a former sergeant with a special forces background and the nickname Rambo, Adam Samia and Carl David Stillwell have denied they planned the 2012 execution-style hit — a case that's provided an inside glimpse into the secret fraternity of private mercenaries willing to kill in cold blood for cash.

Recruited for 'ninja work'

The court was told Mr Hunter, 52, was working as a security chief for weapons and drug trafficker Paul Le Roux when he recruited Mr Samia and Mr Stillwell to travel from their homes in North Carolina to the Philippines for what was called "ninja work".

Assistant US attorney Patrick Egan told the court Mr Hunter provided firearms and silencers and told them Le Roux would pay them $US35,000 ($45,570) each to kill real estate agent Catherine Lee.

Ms Lee was on a "kill list" that self-styled assassins with military backgrounds saw as a golden opportunity, Mr Egan said.

"If Paul Le Roux wanted somebody killed, these guys got the call," he said. "For these men, more murders meant more money."

Body 'found in a pile of garbage'

Mr Samia, 43, and Mr Stillwell, 50, conducted surveillance on Ms Lee before contacting her pretending to be potential clients, the prosecutor said.

While returning from a trip outside of Manila, Mr Samia used a .22 calibre gun to kill Ms Lee by shooting her twice in the face as she sat in the back seat of a van, he said.

Ms Lee's body was found on a pile of garbage on the side of the road, Mr Egan said.

After being paid, her killers were ordered back to the United States, where they were arrested in 2015.

An investigation discovered a picture on Mr Stillwell's phone of a bloody head wrapped in a towel that was taken around the time of Ms Lee's death, authorities said.

Mr Stillwell also admitted being behind the wheel of the van when she was shot, they added.

Prosecutors said other evidence includes a secretly recorded video of Mr Hunter from a sting in Thailand that resulted in a separate conviction for plotting to kill a Drug Enforcement Administration agent.

They said he is overheard talking about killing people for Le Roux, including a Filipino real estate agent.

Jurors asked to keep an open mind

On Tuesday, defence attorneys asked jurors to keep an open mind, saying the case lacked the eyewitnesses, forensic and other conclusive evidence needed to convict the trio.

They also told them not to trust government witnesses like Le Roux, who has pleaded guilty and is cooperating.

Mr Hunter's attorney, Cesar De Castro, suggested the government could not prove its case.

Mr Samia only agreed "to do legitimate, legal security work", said his lawyer, Jeremy Schneider.

Mr Stillwell's lawyer, Robert Ray, didn't deny his client's statements about being in the van but claimed he never joined a murder conspiracy.

"It just didn't happen," Mr Ray said.

AP