SEOUL (REUTERS) - The liberal South Korean politician most likely to become the country's next president would, if elected, review how the government would deploy an advanced US missile defence system and would consult China, two of his top advisers said on Friday (March 17).

If Mr Moon Jae In, the front-runner for the May 9 presidential election, reverses policy on the deployment of the Terminal High Altitude Area Defence (Thaad) missile defence system, it will place him at odds with the United States, South Korea's biggest ally.

The conservative government of impeached president Park Geun Hye agreed to deploy the Thaad system to guard against an attack by North Korea, but the decision sparked outrage in China, which responded with restrictions on some companies doing business with and in South Korea.

China says the system's radar can be used to spy into its territory.

Mr Moon would likely "do a review of the validity of the decision", said Professor Choi Jong Kun at Yongsei University and an adviser to Mr Moon on foreign policy.

"While doing it he will consult with the United States, as well as China," Prof Choi said in an interview with Reuters.

"At the end of the day, if the reality unfolds in a way that South Korea's national security and the economy were damaged because of the Thaad, not because of the North Korea issue, then it's not really a rational situation, is it?"

The comments are at variance with a tough stand taken by the new US administration on North Korea.

US Secretary of State Rex Tillerson, visiting Seoul for the first time since taking office, said on Friday that a US policy of "strategic patience" with North Korea has ended and military action would be "on the table", if North Korea elevated the threat level to warrant it.

Mr Tillerson also said he expected the next South Korean government would "continue to be supportive" of the Thaad system.

China is South Korea's largest trading partner and the dispute over the missile system has left normally bustling shopping districts in the capital Seoul devoid of their usual crowds of Chinese tourists.

In China, the row has led to a freeze of South Korean television dramas and music, and product boycotts.

Mr Moon, a liberal facing little in the way of a significant conservative challenger, said in a debate this week that China should stop the economic retaliation and South Korea had to make diplomatic efforts to assuage Chinese anger. "It's only right for the Thaad deployment issue to be decided by the next administration," Mr Moon told foreign media recently.

A 63-year-old human rights lawyer, Mr Moon has said he will extend an olive branch to North Korea if elected and would visit Pyongyang before making a trip to the United States.

Just two North-South summits have been held since the 1950-53 Korean war.

Prof Choi said the decision to deploy the Thaad battery had been made hastily. China's reaction was foreseeable and yet was largely ignored by Ms Park's government, he said.

"We had a strategic partnership with Beijing, until this Thaad issue," Prof Choi said. "Our relationship had been pretty OK and pretty good."

Professor Kim Ki Jung at Yonsei University, another foreign policy adviser to Mr Moon, said he had tried to convince US military officials and diplomats in Washington last month that the deployment of the Thaad system should be left to the leader who succeeds Ms Park.

"We are going to acknowledge that two governments made an agreement... but the actual process of deployment, that should be given to the next government," he said.

Instead, the United States started to deploy the first elements of the system this month, after North Korea fired off four ballistic missiles into the sea off north-west Japan.

Thaad's job is to intercept and destroy a ballistic missile in its final phase of flight.

Mr Moon has criticised the two former conservative presidents - Ms Park and her predecessor Lee Myung Bak - for derailing progress made in inter-Korean relations during previous liberal administrations.

He calls for a "two-step" approach on North Korea, with talks leading to "economic unification" and ultimately "political and military unification".

His viewpoints could spark friction with Washington, but Mr Moon would have no problem distancing South Korea's interests from those of the United States, said Prof Kim.

"The basic assumption is that we are going to maintain the success of our bilateral alliance," Prof Kim said. "We are going to keep it... as long as we admit that South Korea is not the 51st state of the United States. We are an independent country, we have our own national interest, and we should have our own foreign policy strategy."