Image copyright Reuters Image caption Donald Trump is currently the frontrunner in the Republican race

US presidential hopeful Donald Trump has agreed not to run as an independent candidate if he loses the Republican nomination for the 2016 elections.

"I will be totally pledging my allegiance to the Republican Party and the conservative principles for which it stands," he said on Thursday.

He had earlier refused to rule it out.

The billionaire business mogul, who has been soaring in the polls, has come under pressure in recent weeks from the Republican Party to sign the pledge.

His announcement on Thursday will be seen as a victory for the party, who may have seen a split in its support and given the Democrats a boost had Donald Trump pressed ahead as an independent candidate.

He said he had received nothing for signing the loyalty pledge, aside from the assurance that he would be treated fairly in the race.

'Won't tear it up'

Mr Trump was booed by audience members during the Republican presidential debate last month after he refused to rule out a third-party run. He was the only candidate not to commit to back the winner of the party's primaries.

The Republican Party National Committee has since sought a loyalty pledge from each of its presidential hopefuls, in what is believed to be a first for the party.

"The best way forward... to win, is if I win the nomination and go direct against whoever (the Democrats) happen to put up. So for that reason, I have signed the pledge," Mr Trump told reporters gathered at his campaign headquarters in New York's Trump Tower.

Image copyright AP Image caption Former Florida Governor Jeb Bush attacked Donald Trump this week for being "a Democrat in disguise"

"I see no circumstances under which I would tear up that pledge," he added.

The Republican Party pledge asks presidential candidates to "endorse the 2016 Republican presidential nominee regardless of who it is".

Donald Trump has come under attack from his rivals in the race who have questioned his conservative credentials and liberal leanings in the past.

Former Florida Governor Jeb Bush said on Thursday Donald Trump's views on illegal immigration were "too pessimistic", despite vowing to support his rival if he won the party race.

Some of the measures Mr Trump has outlined to combat illegal immigration include raising visa fees to pay for a wall along the Mexican border and ending the automatic right to citizenship for US-born children of families living illegally in America.

The latest poll by Monmouth University puts Mr Trump way ahead with support from 30% of Republicans, and retired neurosurgeon Ben Carson running a distant second with 18%.

The rest of the Republican pack is trailing far behind, with Jeb Bush currently tied with Texas senator Ted Cruz at 8%.