Former U.S. Defense Secretary Jim Mattis said on Monday anti-government protests in Hong Kong were "not an internal" Chinese matter and that the United States should offer at least moral support to the demonstrators.

The retired U.S. Marine general, speaking at a Reuters Newsmaker event in New York, said the United States should generally side with those standing up for human rights, which he said included the Hong Kong protesters.

"When people stand up for those (rights), I just inherently think we ought to stand with them, even if it's just moral," said Mattis, who abruptly resigned as Pentagon chief in December over disagreements with President Donald Trump's foreign policy.