For Daniel Russel, the top US diplomat for East Asia, the upcoming visit to South Korea by Chinese President Xi Jinping is "an extraordinary milestone".

"It is particularly gratifying for me, personally, since I played a small role in helping to facilitate early contacts between Seoul and Beijing in the beginning of the 1990s when I served at the United Nations," Russel said on Wednesday at a seminar in Washington on the US-South Korean alliance.

The assistant secretary of state for East Asian and Pacific affairs said the US fully supports South Korea's efforts to build strong ties with its neighbors.

"The flourishing relationship between China and South Korea clearly demonstrates that our alliances in the region are a force for stability and integration, and that active US engagement is good for the Asia-Pacific region," he said.

China has been wary of US efforts to strengthen its alliances in recent years, especially with Japan and the Philippines, both of which have maritime territorial disputes with China.

Just last week, Chinese Ambassador to the US Cui Tiankai implied such alliances were a Cold War era strategy. "Any attempt to establish a confrontational military alliance will result in lose-lose outcome," Cui said at a reception in Washington marking the 35th anniversary of diplomatic ties between China and the US.

Cui also made reassurances that China's strategy was not to seek global dominance, let alone challenge or replace any other nation, a bid clearly aimed at easing concerns in the US.

Compared with the US alliances with Japan and the Philippines, the US-South Korea alliance has been much less of a concern to China.

Kim Sung-hwan, South Korea's former minister of foreign affairs and trade and now a professor at Seoul National University, described the speed and scope of the development of the China-South Korea relationship as "unprecedented" since their diplomatic ties were established in 1992.

He said China provides South Korea an opportunity for development, citing the fact that China is the largest trading partner for South Korea and the South Korea's trade with China is larger than its trade with the US and Japan combined.

Xi and South Korean President Park Geun-hye, who speaks Chinese, have met in three summits — in Beijing in June 2013, in Bali in October of the same year and in the Netherlands last March.

Talking about the warming relations between China and South Korea, Russel lamented the strained relations between South Korea and Japan, both US allies.

Japanese Prime Minister Shinzo Abe angered both China and South Korea with his right-wing historical view of whitewashing Japanese atrocities in WWII and his bid to expand the Japanese military. The move reminds many of the Japanese militarism of WWII which inflicted vast suffering on the people of the region.

While praising a dialogue between Japan and South Korea on historical issues, Russel admitted that there was hard work ahead for both sides.

"This cannot be done by one party alone," he said. "And the hard work is made more difficult by politicizing and by the erosion of trust."

Park Cheol-hee, professor at the Seoul National University and a Japan specialist, pointed out that Japan's actions in the region are not proactive. "If you want to promote peace in the region, Japan should make much more effort to have a good relationship with its immediate neighbors. We don't see it," Park said.

The US has found it hard to keep a proper balance between its allies and other countries, such as the recent tensions over maritime territorial disputes in both the South and East China seas.

In Hanoi, Chinese State Councilor Yang Jiechi met with Vietnamese Deputy Prime Minister and Foreign Minister Pham Binh Minh on Wednesday in a bid to manage the tense maritime situation.

Alan Romberg, a distinguished fellow and director of the East Asia program at the Stimson Center in Washington, believes it's not easy for the US to keep its close alliance with Japan and South Korea without antagonizing China.

"But I think it's very important to think that those are not incompatible goals," he said.

He said whether it can be successful depends not only on the US, but on others as well, in particular China. "So I don't consider it ‘mission impossible'," he said.

Robert Hathaway, director of the Asia program at the Wilson Center, said that each of the three bilateral relationships — between Washington and Seoul, Washington and Tokyo, and Washington and Beijing — is immensely important to the US, although not equally important.

"We believe it's so obviously in the interest of all the parties in the region to figure out a way to contain their differences. Differences aren't going to go away. The trick for the diplomats is how to keep differences from spilling over and souring the entire relationship," he said.

Douglas Spelman, a senior advisor at the Kissinger Institute on China and the United States, believes the US needs to be even-handed in its approaches to problems in East Asia.

"It's true that China has pushed a little bit, but other countries have pushed a bit too. I think we need to make sure that that we maintain even-handedness. I think that's the way we can help," he said.