He's not running(Photo by Darren McCollester/Getty Images)

This story was updated at 12:30 p.m.

John Bolton, a former U.S. ambassador to the United Nations, announced Thursday that will not run for the GOP presidential nomination, ending speculation that he would enter the crowded Republican field.

"I have decided not to seek the Republican nomination for president," Bolton said in a video published on his Facebook page. "I believe I can make the strongest contribution to our future by continuing as a clear and consistent advocate for a strong, Reaganite foreign policy that values peace through strength."

The former ambassador's announcement Thursday marks the end of speculation over whether would make a White House bid, which he has fueled with frequent trips to several early voting states. He also briefly considered running for president in 2012.

Bolton, 66, is one of his party's best-known advocates for a robust foreign policy. He has remained a prominent voice on national security issues since leaving the Bush administration in 2006 – most notably as a Fox News contributor – and has been a frequent critic of President Obama’s handling of foreign affairs. Bolton’s decision not to seek the GOP nomination notably comes just as the former president’s legacy in the Middle East has taken center stage in the 2016 election.

The GOP presidential field currently includes six declared candidates with up to a dozen more candidates weighing bids to launch in the coming months. Candidates expected to enter the race in the coming weeks include former Pennsylvania senator Rick Santorum, South Carolina Sen. Lindsey Graham and former Texas governor Rick Perry.

Although he will not seeking his party's presidential nomination, Bolton said that he will be active player in the election, pledging to "make certain foreign policy is critical" in the GOP primary.

“While I am not a candidate, I am certainly not going to sit this election out. As in 2014, I’ll be supporting national security candidates and stressing those issues in the Senate and House campaigns,” he said in the video. “I’m also going to focus on the 2016 Republican presidential race to make certain foreign policy is critical to winning the nomination."