NEW YORK (Bloomberg) - President Donald Trump nominated K.T. McFarland to be US ambassador to Singapore, removing her as a deputy national security adviser less than four months after she was brought on by Michael Flynn.

Trump also announced he nominated Callista Gingrich, the wife of former House Speaker and Trump supporter Newt Gingrich, to be the ambassador to the Vatican.

The president will meet with Pope Francis Wednesday (May 24) in Rome.

McFarland joined the administration under Flynn, who was fired from his role as Trump's national security adviser in February.

She will be replaced at the National Security Council by Ricky Waddell, a major general in the US Army Reserve who served in Iraq and Afghanistan and spent 17 years working in the private sector in Latin America, the White House said on Friday in a statement.

The removal of McFarland has been planned since at least April and was one of a series of moves Flynn's replacement, H.R. McMaster, has been making, including a reorganisation of the National Security Council and the addition of former Goldman Sachs Group executive Dina Powell as a deputy national security adviser.

McFarland, 65, a former Fox News commentator, will replace the most recent US ambassador to Singapore, Florida attorney and Barack Obama fundraiser Kirk Wagar, who departed in January upon Trump's inauguration.

The appointment is subject to Senate confirmation.