China's president flew to Seoul Thursday for a state visit focused on nuclear developments in North Korea, which has spent the past week playing hawk and dove with threats, missile tests and peace offers.

It will be Xi Jinping's first trip as head of state to the perennially volatile Korean peninsula, and his second summit with South Korean President Park Geun-Hye who visited China last year.

North Korean leader Kim Jong-Un is still waiting for an invitation to Beijing -- a perceived snub that speaks to the strained relationship between Pyongyang and its historic and most important ally.

"No previous Chinese leader has put South Korea before and above the North like this," said Aidan Foster-Carter a Korea expert at Leeds University.

In what some saw as a display of pique at Xi's visit, North Korea conducted a series of rocket and missile launches into the Sea of Japan (East Sea) over the past week, triggering protests from Seoul and Tokyo.

The North has been in particularly mercurial rhetorical form of late, one day threatening a "devastating strike" against the South and the next proposing a suspension of all hostile military activities.

South Korea on Tuesday rejected the peace offer as "nonsensical" and suggested that Pyongyang show its sincerity by dumping its nuclear weapons.

Xi and Park will hold their summit after Thursday's official welcoming ceremony, and the two leaders are then expected to sign a joint communique.

- Strong line on North Korea? -

Seoul will be hoping for a strong statement on North Korea's nuclear weapons programme, but analysts said Beijing was unlikely to up the rhetorical ante by any significant degree.

"That would go against China's traditional diplomatic pattern," said Kim Joon-Hyung, professor of politics at Handong Global University.

"Xi will probably keep to the general line of urging the denuclearisation of the Korean peninsula, rather than criticising the North directly," Kim added.

As the North's diplomatic protector and chief economic benefactor, China has repeatedly been pressured by the international community to use its leverage to rein in Pyongyang's nuclear ambitions.

But while Beijing has become increasingly frustrated with the North's missile and nuclear tests, it remains wary of penalising the isolated state too heavily.

It is especially anxious to avoid any regime collapse that would result in a unified Korea with a US troop presence on its border.

Washington has played up Xi's visit as evidence of Pyongyang's deepening diplomatic isolation.

"The symbolism of a visit by a Chinese leader to Seoul against the backdrop of tensions between North Korea and its neighbours ... is pretty striking," US Assistant Secretary of State Daniel Russel told AFP.

The wider background to Xi's trip includes China's response to the US "pivot to Asia" and the battle between the two major powers for regional influence.

China is currently South Korea's largest export market and two-way trade stood at around $275 billion last year, but analysts say Beijing wants to move beyond economic ties and promote political and security links.

This leaves Seoul with a difficult balancing act, given its historic military alliance with the United States.

There are currently around 29,000 US troops stationed in South Korea, which is also protected by the US nuclear umbrella.

So how far would South Korea be willing to go in developing its ties with China beyond the economic sphere?

"Partly it depends who holds power in Seoul," Foster-Carter wrote on the NK News website.

"Conservatives like Park will ensure the US alliance is not weakened, especially while North Korea continues to snarl.

"But South Korean presidents change every five years. If liberals return to power in 2018, the left's neutralist and Yankee-bashing tendencies might come to the fore," he said.

The military ambitions of the other main US ally in the region, Japan, is also likely to figure in Thursday's summit talks, with both China and South Korea concerned by the recent change to its pacifist constitution.

Xi's visit to Seoul 'important milestone,' says US
Washington (AFP) July 02, 2014 - A visit to South Korea this week by Chinese President Xi Jinping marks "an important milestone" in warming ties and is in stark contrast with Beijing's "chilly" relations with Pyongyang, a US diplomat said Wednesday.

Xi is due to visit Seoul on Thursday for talks with his South Korean counterpart Park Geun-Hye on a closely watched state visit which will include discussions on North Korea's nuclear program.

Beijing however, despite being the North's traditional ally, has not had a summit with Pyongyang since the December 2011 death of its then-leader Kim Jong-Il.

"Clearly the net effect of a visit by the Chinese president to the Republic of Korea, that showcases the dramatic warming of relations and the broadening of practical cooperation, stands in pretty stark contrast to the chilly relationship between Beijing and Pyongyang," Assistant Secretary of State Daniel Russel told AFP in an interview.

Beijing and Washington are seeking "to persuade North Korea that its only viable option is to take steps to come into compliance with its international obligations," Russel said.

Six-party talks with Pyongyang on reining in its nuclear program have been stalled since 2009, and the North has carried out a series of rocket launches denounced by the United States as provocative.

On Wednesday North Korea fired two short-range rockets off its east coast, marking the third such test by the North in the past week -- all three involving firing into the Sea of Japan (East Sea).

North Korea is set to be the focus of some "in-depth discussions" when top Chinese and US leaders meet in Beijing next week for annual strategic and economic talks.

Both China and the United States want to achieve de-nuclearization "through negotiations and we have been refining an approach that would ensure that North Korea comes to the table with convincing evidence that it's prepared to truly negotiate."

"No one wants to go back to the merry-go-round of talks for talks' sake," Russel said.

Washington has long been pushing Beijing to use its sway with Pyongyang to bring it back to the negotiating table, and has increased sanctions against the isolated communist-run state.

"The symbolism of a visit by a Chinese leader to Seoul against the backdrop of tensions between North Korea and its neighbors... is pretty striking," Russel said.

"The Chinese have increasingly moved in the direction that denuclearization is more than just a slogan, it is an objective that needs to be implemented, and implemented on a credible and rapid timeline."