A President Donald Trump will neither abandon South Korea nor let the Asian ally defend itself against threats from North Korea, a top foreign policy adviser to the presumptive Republican presidential nominee said.

Walid Phares, an international relations scholar who serves as Trump's foreign policy brain, made the remark in an interview with Yonhap News Agency and Yonhap News Television, stressing that Trump's remarks, such as his call for Seoul to shoulder all costs for U.S. troop presence, should be taken only as an expression of "principles."

Phares also said that Trump wants to "go back to ground zero" when it comes to free trade agreements, including the one with South Korea, and is ready to renegotiate them. But he stressed that renegotiation does not mean cancelling everything.

"Mr. Donald Trump basically wants to be a president who would never abandon his allies on the one hand, and on the other hand, would confront those who are considered as national security threats to this country but to our allies as well," Phares said.

"It is clear in the mind of Mr. Trump, and the mind of the people who elected him that the South Korean people have been standing with America for decades," he said, adding that the U.S. helped South Korea during the Korean War, and the South helped the U.S. in Vietnam, Afghanistan and elsewhere.

That "brotherhood of arms ... cannot be erased overnight" and will continue, he said.

During the interview, Phares sought to calm uneasiness sparked in South Korea, Japan and other allies by Trump's remarks that appeared to show he has deeply negative views of U.S. security commitments overseas as he contended the U.S. should stop being the policeman of the world.

In a media interview earlier this month, for example, Trump said that South Korea should shoulder all costs for the upkeep of 28,500 American troops stationed in the country, and the U.S. should "walk" unless Seoul agrees to do so in an apparent threat to withdraw the troops.

"At this point in time he is a candidate. He's a Republican candidate, practically speaking. So, he will talk about principles, and those principles mean burden-sharing. He wants the South Korean government to share more," Phares said. "There is a principle ... that America alone cannot be defending the world. It does not mean that America wants to withdraw from the world."

Trump will see "what the South Koreans can offer, or what the Japanese would offer, what our friends in the Middle East could offer. Then he would negotiate. He is good at negotiations," Phares said of the call for defense burden-sharing.

Such negotiations do not mean Trump will abandon South Korea, Phares said.

"If South Korea is threatened by either North Korea, or other players, a Trump administration would be standing by the South Koreans. There is no idea that we're going to let South Korea or Japan defend themselves against a threat," he said.

Referring to Trump's call for South Korea to shoulder all costs for American troop presence, Phares said that the real estate tycoon "wants to show the maximum" as a negotiator before settling for reality. And such maximum positions could evolve over time, he said.

"You're going to see more of this evolution over the next weeks and months as more experts are coming around him, as the party is unifying around him," he said. "If he is elected, then the agencies of the U.S. government will be around him and tell him, Mr. President, these are the facts and you make the political choice that you want."

He also said that a troop withdrawal from South Korea is "the last scenario to consider."

"Mr. Trump is not going to jump to the last scenario. This is the last scenario of crisis. But he is mentioning it to the American public that we really want to sit down and have a serious negotiation," he said, adding that Trump's mention of the scenario is "an expression of a negotiator."

Trump has been strongly critical of free trade agreements the U.S. has forged with foreign countries, sparking concern that, if elected, he could seek to break or renegotiate the landmark free trade deal with South Korea.

Phares said that Trump "wants to go back to ground zero in each of these agreements."

"When we say renegotiate it does not mean cancel everything. It means to sit down and see, this is what has happened in America since the time we have negotiated and had the agreement and you tell us what are your cards, and we come closer," he said.

On North Korea, Trump will be adamant about making sure that the communist nation won't become a threat against South Korea, against Japan, and against the region and the international community, Phares said. A Trump administration won't be scared by the North's nuclear threats, he said.

"You're going to see an American president, a new administration, which would be very different from how the Obama administration has dealt with this crisis," he said.

Phares reiterated Trump's strategy for dealing with the North: pressuring China to rein in Pyongyang. He said that the past administration failed to put enough pressure on Beijing to use its leverage over the North, but a Trump administration "will do the job."

He described Trump's policy as a four-circle or four-step policy that calls first for strengthening relations with the South and then with Japan and other regional partners before reaching out to China and Russia for greater pressure on Pyongyang. The last circle would be that if the North is behaving in a very threatening way, the U.S. and its allies will have to show and demonstrate their determination, he said.

Asked if Trump is willing to hold talks with North Korean leader Kim Jong-un, Phares said that Trump is willing to negotiate with "anybody if needed," but such talks won't happen as long as Pyongyang remains aggressive.

"First, there needs to be a change of behavior," he said, adding that once the Soviet Union decided to change its behavior during the Cold War, it got an American position that is ready to negotiate.

"We don't see a Gorbachev in North Korea at this point in time," Phares said, referring to former General Secretary of the Communist Party of the Soviet Union Mikhail Gorbachev, whose policy on reform and openness led to the end of the Cold War. (Yonhap)