U.S. Republican presidential front-runner Donald Trump is willing to strengthen relations with allies, even though he will seek to have them pay more for American defense support, a foreign policy adviser was quoted as saying Sunday.

Walid Phares, an expert on terrorism and Middle Eastern affairs who served as a foreign affairs adviser for former Republican presidential candidate Mitt Romney, made the remark in an interview with the Jiji Press, according to the Japan Times.

Trump "would like to strengthen" the alliance, Phares was quoted as saying.

In a major foreign policy speech last week, Trump said that allies are not paying their fair share for U.S. defense support, and the U.S. must be prepared to "let these countries defend themselves" unless they foot more of the bill for defense.

Though the real-estate tycoon made no mention of South Korea, the remark spurred concern that if elected, he could seek to pull the 28,500 American troops from the Asian ally unless Seoul agrees to pay more.

Trump made the case again on Sunday,

"Look, we're defending Germany, we're defending Japan, South Korea, we're defending Saudi Arabia with all of that money, and we're not getting properly reimbursed," he said on "Fox News Sunday." "We're like the policeman to the world. What's going on is crazy."

He also said that foreign countries are concerned about him because they feel he's very strong, and that he will have much more respect than Democratic presidential front-runner Hillary Clinton from foreign leaders.

Phares said that strengthening alliances is one thing and renegotiating cost-sharing is another.

"When it comes to the defense matters and spending, spending is one track and the alliance is another track," he said in the interview.

"Our commitment to allies ... is going to be permanent," Phares was quoted as saying. "This is part of our policy. If there are intentions by hostile forces against our allies, we will actually be standing with our allies."

Phares said Trump's apparent threat to pull American troops from allies could be "a theoretical scenario," but that does not in any way mean he is going to abandon the alliances.

Trump's remarks mean that he's serious about negotiating cost-sharing.

Phares described Trump's controversial suggestion that South Korea and Japan could be allowed to develop nuclear weapons for self-defense as "extreme scenarios," adding that Trump prefers to have all options on the table. (Yonhap)