China's president on Friday wrapped up a state visit to South Korea that was heavier on symbolism than substance, but also exposed the slowly but steadily shifting bedrock of historical, Cold War alliances in East Asia.

Xi Jinping's trip had been seen as a pointed snub to North Korea -- his decision to visit Seoul before Pyongyang a sign of Beijing's growing frustration with its wayward, unpredictable nuclear-armed ally.

Mao Zedong once declared China and North Korea to be as close as "lips and teeth" -- a bond forged in the 1950-53 Korean War against the South and US-led UN forces.

But Beijing's patience with the North's relentless nuclear brinkmanship has worn thin and Xi's visit was a clear reflection of the common ground it now shares with the South -- economically and, to a growing extent, diplomatically.

If South Korea hoped this might all translate into a joint, strongly-worded warning to the North over its nuclear programme, it was disappointed.

The statement that emerged from Xi's summit with South Korean President Park Geun-Hye on Thursday didn't even reference "North Korea" directly, calling instead for the "denuclearisation of the Korean peninsula" -- a formulation long-favoured by Beijing.

Rather than North Korea, Xi ended up expressing common cause with South Korea over an older regional rival -- Japan.

In a speech at Seoul National University on Friday, he recalled the "barbarous wars of aggression" Japan had waged against China and Korea and the suffering inflicted by occupation and colonial rule.

Such rhetoric plays well in the South where relations with Japan are at their lowest ebb for years, mired in disputes related to Japan's 1910-45 rule over the Korean peninsula.

The rift is a source of increasing anxiety for the United States, whose strategic "pivot" to Asia is on a more fragile footing with its two main military allies in the region barely on speaking terms.

For China, however, it's an opportunity, according to Lee Shang-Hyun, a senior researcher on security and international relations at the Sejong Institute think-tank in Seoul.

- Exploiting rifts -

"A prolonged rift between Japan and South Korea will obviously help China extend its influence over the Korean peninsula, and its diplomatic goal of driving a wedge into the trilateral alliance involving Seoul, Tokyo and Washington," Lee said.

"There won't be any fundamental change in the diplomatic landscape in the short term, but the situation is more fluid than it was, with countries like Japan and South Korea seeking to maximise their own interests as China and the US compete for influence in the region," he said.

And Japan showed this week that it had some cards to shuffle in any new geopolitical deck.

On Tuesday, Prime Minister Shinzo Abe announced a highly contentious shift in Japan's pacifist military policy, asserting its right to go into battle in defence of its allies -- a move viewed with deep suspicion in Beijing and Seoul.

And two days later, even as Xi arrived in Seoul, Japan announced the unilateral lifting of selected sanctions on North Korea as the result of progress in talks with Pyongyang over its Cold War kidnapping of Japanese nationals.

North Korea and Japan are unlikely partners -- anti-Japanese sentiment in the North is almost as high as it is in the South -- but Pyongyang, like Beijing, is always happy to shake the Tokyo-Seoul-Washington alliance.

In the end, a lot of messages were sent around the region and beyond this week, reflecting the more nuanced and complex geopolitical landscape being laid over the old certainties and alliances of the Cold War.

There was one nod to diplomatic tradition though, as China announced it was marking Xi's visit to South Korea with the gift of two panda bears.

But even gifts are subject to changing fortunes.

After China made a similar present of two pandas in 1994, South Korea was forced to return them four years later when the Asian financial crisis struck and it was unable to afford their costly upkeep.