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.

Doubling down on his pre-inauguration threat to reimpose U.S. control over the canal, Trump on Monday accused Panama of breaking the promises it made for the final transfer of the strategic waterway in 1999 and of ceding its operation to China - claims that the Panamanian government has strongly denied.