SEOUL, South Korea - As he hops around the Western Pacific this week, President Obama hopes to unite much of Asia around a free-trade deal, updated alliances, and a new power balance. But he first must persuade two of America's closest allies to stop squabbling.

Jetting from Tokyo to Seoul on Friday morning, his second stop on the trip, Obama was between two nations mired in an old feud. South Koreans are furious over what they perceive as inadequate remorse from Japan over its brutal colonization of their nation from 1910 to 1945 and its use of Korean "comfort women" as sex slaves during World War II.

Japanese Prime Minister Shinzo Abe and Korean President Park Geun Hye have traded slights and diplomatic digs for months. Tension is so high that a recent poll found Abe as unpopular among South Koreans as North Korea's strongman, Kim Jong Un.

Obama will spend Friday laying a wreath at South Korea's national war memorial, visiting a historic palace in Seoul, and trying to ease the tension between Park and Abe, who was Obama's host for two days in Tokyo. At stake is coordination on key security and economic issues in northeastern Asia, where North Korea threatens both Japan and South Korea.

Obama's trip has already provided one disappointment. As he prepared to leave Tokyo, negotiators ended talks on the proposed Trans-Pacific Partnership, a 12-nation trade deal, without the breakthrough White House aides had wanted.

The politics of protocol will only get tougher. In particular, Obama wants to avoid offending China, embroiled in maritime disputes with most of its neighbors.

In Tokyo, Obama affirmed the U.S. security treaty with Japan, but he emphasized the need to work to resolve territorial differences in a peaceful manner.

"Not escalating the situation, keeping the rhetoric low, not taking provocative actions, and trying to determine how both Japan and China can work cooperatively together," he said, characterizing the advice he gave Abe behind closed doors. "And I want to make that larger point. We have strong relations with China. They are a critical country not just to the region but to the world."