The United States is looking for a new national security adviser after the departure of John Bolton, who left the White House on Monday following a series of disagreements with President Donald Trump. 

The president is yet to permanently fill several senior positions in his administration, but tweeted on Monday evening that he would announce a replacement for Bolton "next week." The new appointment could signal a fresh approach to foreign policy ahead of the 2020 election.

Here are some possible contenders to replace Bolton:

Stephen Biegun

Biegun is currently the U.S. Special Representative for the Democratic People's Republic of Korea. Known more as a technocrat than an ideologue, the 56-year-old has worked closely with Trump and Secretary of State Mike Pompeo on a diplomatic solution to tensions on the Korean Peninsula. Bolton, by contrast, favored a bellicose approach. He served as an executive secretary of the National Security Council under President George W. Bush.

Brian Hook

Hook is currently the U.S. Special Representative for Iran and a senior policy adviser to Pompeo. The lawyer was appointed during Rex Tillerson's time at the State Department, and had been expected to become increasingly involved in Jared Kushner's Middle East plan. Senator Lindsey Graham, a Trump ally, told Fox News that Hook was under consideration. 

Richard Grenell

Grenell, the current U.S. ambassador to Germany, has a close relationship with Trump and has been linked with several high-profile vacancies over recent years. Fox News' White House correspondent Kevin Corke tweeted that Grenell was one of the names in the frame to replace Bolton.

Keith Kellogg

Kellogg, who served in Trump's National Security Council under H.R. McMaster, is currently national security adviser to Vice President Mike Pence. He was mentioned by Graham as a possible replacement for Bolton.

Charles Kupperman

Kupperman is Bolton's acting replacement, having served as deputy national security adviser since January. The 68-year-old serving under Bolton may be a disadvantage in getting the job permanently, but the New York Times reported that Trump favorably contrasted his "just-the-facts style" with Bolton's ideology-driven approach.

H.R. McMaster

McMaster served as Trump's second national security adviser, but was fired after comments about alleged Russian interference in the 2016 presidential election. The men had a fractious working relationship, yet NBC has reported the president "missed" the former lieutenant general and called him repeatedly over recent months.

Ricky Waddell

Waddell served as deputy national security adviser to McMaster and is now assistant to the Chairman of the Joint Chiefs of Staff. Steve Nelson of the Washington Examiner tweeted that Waddell was one of the names in the frame.

Jack Keane

The retired four-star general has been an informal adviser to Trump on security matters throughout his presidency. But having twice turned down the job of secretary of defense, he is unlikely to be willing to take on a formal role in the White House.

Douglas Macgregor

The former U.S. Army colonel is a Fox News regular, as Bolton was before his appointment, and has called for American troops to withdraw from Afghanistan. CNN reported that his name had been discussed by the White House. 

Peter Hoekstra

Hoekstra, the U.S. ambassador to the Netherlands, has been linked to the roles of national security adviser and director of national intelligence. He spent seven years on the House Intelligence Committee.

Rob Blair

Blair is currently national security adviser to acting chief of staff Mick Mulvaney, whose influence could put the former Office of Management and Budget aide in contention.

Robert O'Brien

O'Brien, the president's envoy for hostage affairs, hit the headlines after claims he described Trump as the "greatest hostage negotiator this country has ever had." Bloomberg reported that he was being considered as the next national security adviser.