Riding on a bullet-proof train, North Korean leader Kim Jong-un arrived earlier this week in Beijing, just in time to spend his 35th birthday alongside Chinese President Xi Jinping.

The surprise trip to the Chinese capital served as a reminder to Washington that nuclear-armed Pyongyang has other strategic allies, underscoring the importance of the Sino-North Korean relationship before Kim's possible second summit with US President Donald Trump.

"It's no secret China has always wanted to be a part of this talk," said James Kim, director at the Asan Institute for Policy Studies in South Korea's capital Seoul.

He added that the two-day trip shows that Kim is "sensitive" to the demands of China, North Korea's primary trade and aid source.

But it may also have sent a message to Trump that Pyongyang has other options if rapprochement with Washington fails.

Focus on sanctions

At their high-profile summit in Singapore last year, Kim and Trump had signed a vaguely-worded pledge on denuclearisation but progress between the two sides has since stalled amid disagreements over the interpretation of their agreement.

In his annual New Year's speech last week, Kim renewed his commitment to denuclearisation of the Korean Peninsula but warned the United States of taking a "new path" if it doesn't ease sanctions.

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His trip to Beijing offers a glimpse as to where that path could lead.

Pyongyang could be using the visit to show the US that it has an "alternative power to gravitate towards", according to Tai Wei Lim, a research fellow at the National University of Singapore.

The North Korean leader is eager to achieve sanctions relief, but he seems to know Xi may be better able to grant that wish than Trump.

The US president's strategy of maximum pressure depends largely on cooperation from Xi. With over 90 percent of North Korea's trade volume flowing through China's borders, Xi can control the faucet of North Korea's economy better than Trump by choosing to enforce those sanctions or take a more relaxed approach.

Recently, China may be doing more of the latter, says James Kim, pointing to ongoing rumours of "de facto loosening of sanctions implementation in China". And while Beijing hasn't formally claimed to be ignoring enforcement, reports about "pockets of sanctions violations" are growing, he added.

'Have his cake and eat it too'

North Korea has a well-honed talent for playing the world's major players against one another, leading analysts to believe that Kim at this point may be using that skill for leverage to get what he wants from a second summit with Trump: both striking a deal with the US and keeping his nuclear weapons.

"He wants to have his cake and eat it too," argues James Kim.

North Korea has been under international sanctions fairly consistently since 2006, curtailing its resource-based economy by capping exports on coal and banning exports of copper and nickel. China has also banned imports of textiles from the North, while several countries have an embargo on selling them luxury goods.

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The campaign on sanctions may be losing steam though, with North Korea importing $640m in luxury goods from China in 2017. Rumours of thinly disguised coal shipments to South Korea have added to US frustrations in getting Kim's neighbours to maintain pressure.

If Beijing is willing to loosen its grip on sanctions, Kim could indeed emerge as a winner. But China, perhaps hesitant to sour relations with Washington amid a bruising trade war between the world's two biggest economies, may also be hoping to extract something from Kim's visit.

Xi seeks an end to the ongoing trade dispute, which is hurting the Chinese economy. Experts agree that while neither the US nor China is "winning" in the trade war, China may be at a disadvantage.

By inviting Kim to meet him in China before a summit with Trump, Xi may be sending a message to Washington that "Beijing still has leverage over Pyongyang", according to Lim.

The fact that Kim's visit came at the same time US negotiators were in China to discuss an end to the trade war is also unlikely to be a coincidence, said Sangsoo Lee, head of the Stockholm-based Korea Center at the Institute for Security and Development Policy, adding that there could be "political intention" behind it.

If Trump believes Xi holds the key to Kim's cooperation, the Chinese president will have another card to play in trade talks. The two sides are wrangling to draft a deal before March 2, when US tariffs on Chinese goods are expected to intensify if no agreement is reached.

Vagueness as strength?

Kim has said openly that his goal remains modernising the North Korean economy, which he underlined by visiting a traditional Chinese medicine factory in Beijing on Wednesday.

In his address to the nation last week, Kim outlined goals to upgrade North Korea's industrial capabilities, especially in pharmaceuticals. The country is a major producer of ginseng, a common ingredient in Chinese medicines, highlighting a potential export opportunity for Pyongyang.

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But so far Kim's strength has been in what he doesn't say.

Like his father - Kim Jong-il, who ruled North Korea from 1994 to 2011, Kim's strength has always been his "vagueness" about what he'll do next and an ability to "keep everybody guessing until they do it", said James Kim.

While Trump is happy to broadcast his plans and successes on Twitter, Kim's skill as a negotiator lies in not showing his cards "until the very last moment", he added.

Instead, it may be more fruitful to look at what Pyongyang has actually done so far in the wake of the Singapore summit.

"Pyongyang has taken no meaningful steps towards denuclearisation," said Bruce Klingner, a senior research fellow at The Heritage Center in Washington.

In fact, according to Klingner, any steps North Korea is taking appear to be in the wrong direction. Satellite images and intelligence reports show North Korea "has continued nuclear and missile production", even expanding their operations, upgrading equipment such as reentry vehicles and mobile missile launchers, Klingner added.

What those actions could show, and what Kim communicated in his New Year's address, is that North Korea is "not really serious about denuclearisation", said James Kim, or at least not in the way the US would like to see it.

"That seems to be fairly clear and out in the open now."

Until now, Kim appears to have been able to get a lot by giving very little. He's had a summit with a US leader, developed a nuclear programme and is now eyeing greater economic expansion through China. And after rallying support for sanctions relief in Beijing, Kim will be hoping to use that leverage to continue his success at a second meeting with Trump.

The question now is how far the US is willing to concede from their original goal of complete denuclearisation, and what its response will be if North Korea chooses a better offer.