US President Donald Trump said on Sunday the US military could deploy troops to Nigeria or carry out air strikes to stop what he called the killing of large numbers of Christians in the West African country.

Asked if he envisioned troops on the ground or air strikes in Nigeria, Trump told reporters aboard Air Force One: "Could be. I mean, other things. I envisage a lot of things. They're killing record numbers of Christians in Nigeria ... They're killing the Christians and killing them in very large numbers. We're not going to allow that to happen."

Trump made the comment on Sunday evening as he headed back to Washington after a weekend at his vacation home in Florida.

Trump threatened on Saturday to take military action against Nigeria if Africa's most populous country failed to crack down on the killing of Christians.

The US president's threat of military action came a day after his administration added Nigeria back to a "Countries of Particular Concern" list of nations that the US says have violated religious freedoms. Other nations on the list include China, Myanmar, North Korea, Russia and Pakistan.