US Senator Lindsey Graham has stated that he will run for president of the United States, pledging to make national security the focus of his campaign.

Graham, a Republican from South Carolina, officially announced on Monday that he has entered the 2016 presidential race.

"I’m pretty sure no one here, including me, ever expected to hear me say, I’m Lindsey Graham, and I’m running for president of the United States," he told a crowd in Central, South Carolina.

"I have more experience with our national security than any other candidate in this race. That includes you, Hillary," he said, referring to Democratic presidential frontrunner Hillary Clinton.

"I know the players, I know our friends and I know our enemies alike -- but most importantly, ladies and gentlemen, they know me," he added.

The war hawk also called for wider military engagement in the Middle East.

Graham's entry further swells the crowded Republican field, whose candidates will increasingly battle for not only votes but fundraising dollars.

On the Democratic field, former secretary of state Hillary Clinton, Vermont Senator Bernie Sanders and former Maryland governor Martin O’Malley are the only ones who have officially declared their candidacy.

The presidential election in the US, held every four years, is essentially a race between a Democrat and a Republican, who win their party's nomination, although candidates from smaller parties can also run.

The next president will take office in early 2017, when President Barack Obama's second term ends.

AHT/HRJ