Panama has complained to the United Nations over US President Donald Trump's "worrying" threat to seize the Panama Canal, even as it launched an audit of the Hong Kong-linked operator of two ports on the interoceanic waterway.

Panama's President Jose Raul Mulino has denied that any other nation was interfering in the canal, which he said was operated on a principle of neutrality. "The canal is and will remain Panama's," Mulino said in response to Trump's threats.