© South China Morning Post
"The aircraft was providing intelligence, surveillance and reconnaissance support at the request of our Philippine allies," the Indo-Pacific Command said in a statement about the crash on Mindanao island.
"The incident occurred during a routine mission in support of US-Philippine security cooperation activities."
The US military said one service member and three defence contractors were killed in the incident. They were not immediately identified pending notification of their families.
"We can confirm no survivors of the crash," the statement said.
Small numbers of American troops are put on short-term rotational deployments in the Philippines, where the US military has helped provide intelligence to troops battling militants linked to Islamic State that remain active on Mindanao.
The Philippine military said in a statement it could not release information about the crash as the matter was classified and an investigation was continuing.
Regional police spokesman Jopy Ventura said officers had not yet determined the cause of the fixed-wing aircraft's crash on a farm near the municipality of Ampatuan.
Police and soldiers were deployed to the site to prevent potential tampering with evidence, the spokesman said.
The plane's tail number, identified by police as N349CA, was registered to defence firm Metrea, according to flight-tracking site FlightAware, which identified it as a Beechcraft Super King Air B300.
The Metrea website describes the company as a "leading provider of effects-as-a-service to national security partners across multiple domains and over a dozen mission areas".
Municipal rescuer Rhea Martin said her team had found four dead bodies at the crash site.
"The bodies were found near the plane," she said, adding: "The plane was cut in half."
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China spends a lot of time and diesel fuel 'exercising' in the Celebes Sea, which is the closest body of water to this American " intelligence, surveillance and reconnaissance" aircraft downing.