Chinese President Xi Jinping, right, and his South Korean counterpart Park Geun-hye, greet children during a welcome ceremony in Seoul on Thursday. Associated Press

China and South Korea pledged to deepen ties on the same day as Japan said it would ease some sanctions against North Korea, twin moves that reinforced the shifting dynamics in a region in which both Beijing and Tokyo are seeking to assert more power.

In Seoul, Chinese President Xi Jinping and South Korean President Park Geun-hye stood together Thursday saying they would seek to complete a bilateral trade pact this year and provide South Korean investors with better access to Chinese financial markets.

Mr. Xi's state visit to South Korea comes the same week as Japan's decision to expand its military role in the region, a move seen warily by both Seoul and Beijing. The trip also upends decades of tradition in which Chinese leaders have always visited longtime ally North Korea first. More than a year in power, Mr. Xi has yet to visit Pyongyang.

Hours earlier in Tokyo, Japanese Prime Minister Shinzo Abe announced a deal with North Korea to ease sanctions on the rogue state in return for an investigation into Japanese citizens abducted by the North decades ago. Though Tokyo still plans to adhere to U.N. sanctions against North Korea, the decision comes as the international community led by the U.S. and South Korea is seeking to maintain a united front in their condemnation of the North for its nuclear and weapons development programs.

Some analysts said the deals reflected maneuvering over long-term strategic and economic goals rather than any sudden or fundamental changes in position. Nevertheless, Thursday's events marked a rare instance when a shifting regional order appeared on clear display.

The diplomatic jockeying is also likely to affect America's own alliances in Asia. By visiting Seoul, Mr. Xi is seen by diplomats and analysts as seeking to capitalize on poor relations between South Korea and Japan to further disrupt ties between the two main U.S. allies in the region.

"China senses an opportunity to draw South Korea away from the United States and Japan, and will use its growing economic clout to underscore its primacy in East Asia," said Frank Jannuzi, president of the Mansfield Foundation.

Asked on Wednesday about the possibility of Japan's lifting of sanctions, U.S. State Department spokeswoman Jen Psaki declined to comment but said the U.S. continued to "support Japan's efforts to resolve the abductions issue in a transparent manner." Meanwhile, China's Foreign Ministry said Thursday it welcomed any thaw that might result from Tokyo's plan.

A repositioning has been under way for several years, as nations have sought to negotiate their roles amid China's global rise. Over the past decade, Japan's economic malaise encouraged South Korea to build up economic ties with China, now Seoul's biggest trading partner. Under Ms. Park, South Korea has also come to see Beijing as a partner, rather than a hurdle, in dealing with North Korea.

As for Pyongyang, which has long viewed Japan as a sworn enemy, analysts say worsening ties with both Beijing and Seoul have prompted it to turn to Tokyo in its hunt for much-needed cash.

Damaged relations between Seoul and Tokyo were once again on display Thursday. South Korea warned Japan over its deal with North Korea, saying moves to lift sanctions "should not hurt international coordination among South Korea, the United States and Japan over North Korea's nuclear and missile programs."

In addition, a Japanese government spokesman called Ms. Park's criticism a day earlier of Japan's move to review how a key wartime apology had been worded "extremely unfortunate."

The visit to Seoul by Mr. Xi comes as Beijing has grown concerned over stability in its backyard, including ongoing provocations by Pyongyang. North Korea has fired a series of short-range missiles from its coast into the sea in recent days and on Thursday vowed to continue doing so despite its now-routine breach of a United Nations' ban on ballistic missile tests.

China's territorial dispute with Japan and Tokyo's moves to reduce restrictions on the deployment of its military have also led it to raise pressure over historical grievances tied to Japan's past as a colonial power.

In the latest move, China on Thursday announced plans to publish online handwritten confessions of 45 Japanese convicted as war criminals in China following the end of World War II. The official Xinhua news agency on Thursday quoted a state archivist as saying the photos were meant to serve as a reminder of Japan's past aggressions.

Meanwhile, Mr. Abe's decision to lift sanctions against North Korea reflects his determination to resolve the abduction dispute, an issue popular among domestic voters and one Mr. Abe identified as a top foreign-policy priority as he took power a year and a half ago.

To be sure, some long-held positions aren't changing. Following their summit talks, Mr. Xi and Ms. Park showed their fundamentally differing approach to tackling North Korea's nuclear program. The Chinese leader emphasized the resumption of dialogue on offering Pyongyang incentives to abandon its atomic weapons, while South Korea is first seeking action from North Korea to show it is sincere about ending its nuclear ambitions for good.

China also appears unlikely to ramp up pressure on North Korean leaders despite its closeness to Seoul. Few believe its long-standing policy of trying to ensure North Korea doesn't implode and destabilize the region has changed in any meaningful way.

"Noncollapse of North Korea remains paramount for Beijing. It will never dance to Washington's tune, much less Seoul's," said Aidan Foster-Carter, an expert in both Koreas and honorary senior research fellow at Leeds University in the U.K.

South Korea, which relies on the U.S. for its defense from North Korean attack, is also eager to emphasize that its closeness to Beijing doesn't come at the expense of its defense alliance with the U.S., which stations 28,500 troops in the country.

At their summit meeting, South Korea and China agreed to speed up free-trade talks and open a direct won-yuan trading market to boost bilateral trade. South Korean institutional investors were also granted an 80-billion-yuan quota to invest in the Chinese equities market.

—Kwanwoo Jun in Seoul contributed to this article.

Write to Alastair Gale at, Yuka Hayashi at and Brian Spegele at

Corrections & Amplifications

The first name of Leeds University Koreas expert Aidan Foster-Carter was incorrectly given as Adrian in a previous version of this article.