In a carefully written post on social media platform Telegram, Volodymyr Zelenskyy accused the Russian army of deploying Chinese citizens on the battlefield in eastern Ukraine.

What about the proof? Well, the Ukrainian president says his security services captured two people from China in the Donetsk region – along with identity documents, personal data and their bank cards.

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A video of a man in military fatigues who had been captured by Ukraine was pinned to the bottom of the statement.

We get snippets of a conversation where the alleged combatant seems to be talking about the events that led to his capture.

“When we arrived at the place… and then my commander.” The man gestures at the floor and ceiling, making shooting noises. “I was also injured.”

These details will make it difficult for the Chinese government to deny the incident out of hand, although they are highly unlikely to supply additional information.

Important details like, who are they? What function(s) do they fulfil in Ukraine’s occupied territories?

Were they volunteers – or mercenaries – who had signed up to fight in the Russian army on their own?

Alternatively, does the Chinese government sanction their involvement – or even encourage it?

That would make the situation far more serious.

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The capture of these Chinese nationals in Donetsk begs another question – how many Chinese are actually fighting for Russia? In his post, Mr Zelenskyy said “there are many more Chinese citizens” other than these two.

Still, the Ukrainian president works hard to suggest the Chinese, officially at least, are blameless.

“Russia’s involvement of China in this war… is a clear signal that [Russian President Vladimir] Putin is going to do anything but end the war,” he writes on Telegram.

This is a generous interpretation of China’s approach to the conflict, which is quite openly contradictory.

On one hand, Chinese President Xi Jinping describes China as a neutral party to the conflict, while simultaneously offering Mr Putin long-term political and economic support.

In fact, he described their partnership as a “no limits” one in a phone call with Mr Putin on the third anniversary of Russia’s full-scale invasion of Ukraine.

Mr Zelenskyy then, is making a point with this post – but he does not want to make the situation any worse.

The severity cannot be overstated, if an additional 50% tariffs are levied on all Chinese goods it will decimate trade between the world’s two biggest economies.

Remember, 50% would sit on top of what is already on the table: 34% announced last week, 20% announced at the start of US President Donald Trump’s term, and some additional tariffs left over from his first term in office.

In total, it means all Chinese goods would face tariffs of over 100%, some as high as 120%.

It’s a price that makes any trade almost impossible.

China is really the only nation in the world at the moment that is choosing to take a stand.

While others are publicly making concessions and sending delegations to negotiate, China has clearly calculated that not being seen to be bullied is worth the cost that retaliation will bring.

The real question, though, is if the US does indeed impose this extra 50% tomorrow, what could or would China do next?

It has said it will “fight to the end”, but what does that mean?

In reality, there are few good options.

There are some obvious measures that China will almost certainly enact.

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Further export controls on rare earth minerals (crucial for the development of high-tech products) are one example. China controls a huge proportion of the world’s supply, but the US would likely find workarounds in time.

Hiking tariffs on high-impact US products such as agricultural goods is another option, but there is only so far this could go.

The potentially more impactful options have significant drawbacks for Beijing.

It could, for instance, target high-profile American companies such as Apple and Tesla, but this isn’t ideal at a time when China is trying to attract more foreign investment, and some devaluation of the currency is possible, but it would also come with adverse effects.

Other options are more political and come with the risk of escalation beyond the economic arena.

In an opinion piece this morning, the editor of Xinhua, China’s state news agency, speculated that China could cease all cooperation with the US on the war against fentanyl.

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This has been a major political issue for Mr Trump, and it’s hard to see it would not constitute some sort of red line for him.

Other options touted include banning the import of American films, or perhaps calling for the Chinese public to boycott all American products.

Anything like this comes with a sense that the world’s two most powerful superpowers might be teetering on the edge of not just a total economic decoupling, but cultural separation too.

There is understandably serious nervousness about how that could spiral and the precedent it sets.

South Korea’s military said its soldiers fired warning shots at North Korean troops who crossed the demarcation line between the two countries.

Around 10 North Korean soldiers violated the military line on Tuesday, but returned after it made warning broadcasts and fired warning shots, South Korea’s military said.

In a text sent to reporters, South Korea’s Joint Chiefs of Staff (JCS), said “North Korean soldiers crossed the military demarcation line (MDL) in the eastern area of the demilitarised zone (DMZ) around 5:00 pm local time.

“Our military is closely monitoring the North Korean military’s activity and taking necessary measures according to the operational procedures.”

Some of the North Korean soldiers were armed, according to the South Korean military.

There is no clear motive for the crossing, but tensions between the two countries have been running high as North Korean leader Kim Jong Un continues to order missile tests and align with Russia over President Vladimir Putin’s war on Ukraine.

North Korean special operations units are among the thousands of troops that Pyongyang has sent to Russia to fight in the Ukraine war, according to South Korea.

In January, Pyongyang tested a hypersonic missile of intermediate range, in a sign of its determination to continue its weapons development programme.

Since re-entering the White House, US President Donald Trump has said he would reach out to North Korean leader Kim Jong Un again to revive diplomatic efforts begun during his first term.

The secretive regime has not responded to Mr Trump’s remarks, but recently claimed US hostilities against it had deepened since the inauguration.

Last month, Mr Kim’s sister threatened the Trump administration with retaliatory action for stepping up “provocations” with the deployment of a US aircraft carrier to South Korea.

Bloodshed and violent confrontations have occasionally occurred at the Koreas’ heavily fortified border, called the Demilitarised Zone, or DMZ.

But when North Korean troops briefly violated the border in June last year, it didn’t escalate as South Korean officials realised they were carrying construction tools and decided the incursion was accidental.

The 155-mile (249km)-long, 2.5-mile (4km)-wide DMZ is the world’s most heavily armed border.

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Around two million mines are inside and near the border, which is also guarded by barbed wire fences, tank traps and combat troops on both sides.

It’s a legacy of the 1950-53 Korean War, which ended with an armistice, not a peace treaty.

The incursion comes as South Korea copes with a leadership vacuum after the ousting of President Yoon Suk Yeol last week over his ill-fated imposition of martial law.