TOKYO -- When President Barack Obama arrives in South Korea on Friday, he will be thrust anew into the role of consoler in chief in a time of crisis, a responsibility he has become all too accustomed to in the United States.

South Korea is reeling from the ferry disaster that has left more than 300 dead or missing, with the vast majority of the victims students from a high school near the capital of Seoul. The tragedy has consumed South Korean President Park Geun-hye in the lead-up to Obama's visit and could distract from the security and economic agenda she had been expected to highlight during her meetings with the U.S. president.

Another world issue is shadowing Obama on his four-nation Asia trip: the crisis in Ukraine. In Tokyo on Thursday, the president said at a news conference that the U.S. has a new round of economic sanctions against Russia "teed up" if the Russians provoke further instability in Ukraine.

White House officials said Obama did not plan to change his schedule in South Korea as a result of the disaster. But the president probably will balance his expected statements -- warnings against North Korean nuclear provocations and calls to lower tensions in regional territorial disputes -- with words of condolence for the ferry victims and the people of South Korea.

Ahead of his trip, Obama said he planned to reaffirm that "our commitment to South Korea is unwavering in good times and in bad."

In an interview published Friday by the South Korean newspaper JoongAng Ilbo, Obama noted that U.S. military forces were involved in the search and rescue effort in the ferry sinking and that his visit to South Korea "will be an opportunity to express the sympathy of the American people. When our friends are in trouble, America helps, and we'll continue to do everything we can to stand with our Korean friends at this difficult time."

The president's trip will come at a sensitive point in the ferry recovery mission, as officials weigh when to bring in cranes and begin cutting up and raising the submerged vessel. More than 140 people are still unaccounted for.

The April 16 disaster has outraged many in South Korea. Most of the ferry's 29-member crew survived, and 11, including the captain, have been arrested or detained in connection with the investigation. Park, the South Korean president, said the actions by some of the crew were "tantamount to murder."

Throughout his five years as president, Obama has been called upon frequently to offer reassurance following natural disasters and other tragedies at home, including twice just this month. On his way to Asia, Obama met with families of the more than three dozen people who perished in a mudslide in Washington state. And in mid-April, he spoke at a memorial service for three victims of shootings at Fort Hood, Texas -- the second time the president has mourned the loss of life in violence at that military base.

Obama will arrive in South Korea on Friday afternoon, local time, from Japan, where he was feted during an official state visit and attended meetings with Prime Minister Shinzo Abe. Ahead of Obama's departure from Tokyo, negotiators from the U.S. and Japan worked through the night to try to resolve differences over a stalled trans-Pacific trade agreement. But Japan's economy minister and chief TPP negotiator Akira Amari said Friday that there had been no breakthrough, though the two sides agreed to continue talks soon.

"Overall, we have narrowed differences, but there is no agreement as a whole," Amari said. The two sides are still struggling to work on few sectors, especially agricultural products and auto, he said.

The president's overnight stay in Seoul is the second stop on a four-country Asia swing that also includes visits to Malaysia and the Philippines.

The president has been serving as something of a mediator between Japan and South Korea, two U.S. allies with strained relations due to Seoul's lingering resentment over Japanese actions during World War II. In March, Obama hosted a trilateral meeting with Abe and Park on the sidelines of a nuclear security summit in the Netherlands, and he has been expected to follow up on that discussion in his individual talks with both leaders this week.

In addition to a meeting and news conference with Park, Obama will also participate in a wreath-laying ceremony at a memorial to the victims of South Korea's many wars, and he will visit the Gyeongbok Palace. On Friday, he'll receive a military briefing from U.S. officials at Yongsang Garrison, then speak to American troops stationed in the region.

As with each of Obama's previous trips to South Korea, the White House is closely watching activity at North Korea's nuclear test site. Commercial satellite imagery showed increased action there this week, but not enough to indicate that an underground atomic explosion was imminent, officials at the U.S.-Korea Institute at Johns Hopkins University said this week.

North Korea last month threatened to conduct its fourth nuclear test, and there has been speculation it may take that step during Obama's trip. The president on Thursday called North Korea "dangerous" and said he was not optimistic of a major shift in Pyongyang's attitude anytime soon.

In the interview with JoongAng Ilbo, Obama said, "If North Korea were to make the mistake of engaging in another nuclear test, it should expect a firm response from the international community. South Korea, Japan and the United States will stand united."

He did specify what that response would be, but said the United States was committed to the defence of its allies, including South Korea, and would work closely with its allies and partners to increase the pressure on North Korea.

"North Korea can meet its obligations, relinquish its nuclear and ballistic missile programs and come into compliance with its international commitments," the president said. "That is the only way that North Korea can obtain the respect, lasting security and economic progress it seeks."

The president will also reiterate his plea for Asian nations to avoid escalating multiple territorial disputes with China. Seoul's key concern is over an area in the East China Sea that is effectively controlled by South Korea but falls within a controversial air defence zone China established last year.