"China repeatedly threatens, more or less openly, to unilaterally change the status quo and shift borders in its favour," Johann Wadephul said in Japan, citing China's behaviour in the Taiwan Strait and the East and South China Seas.

"Any escalation in this sensitive hub of international trade would have serious consequences for global security and the world economy," Wadephul said after talks with Japanese counterpart Takeshi Iwaya.

A statement issued on Sunday before Wadephul's visit to Japan -- and later Indonesia -- said that China was "increasingly asserting its regional supremacy and, in doing so, is also questioning principles of international law."

"China's increasingly aggressive behaviour in the Taiwan Strait and the East and South China Seas also has implications for us in Europe: fundamental principles of our global coexistence are at stake here," the statement quoted Wadephul as saying.

In the joint press statement in Tokyo, Wadephul also criticised "China's support for the Russian war machine" in Ukraine.

"Without it, the war of aggression against Ukraine would not be possible. China is Russia's largest supplier of dual-use goods and Russia's best oil and gas customer," Wadephul said.

He also said ahead of talks later Monday between US President Donald Trump, Ukrainian President Volodymyr Zelensky and European leaders that security guarantees for Kyiv were "crucial".

Trump's summit on Friday with Russian counterpart Vladimir Putin in Alaska "made it clear that for a just and lasting peace, Moscow must finally act. Until that happens, pressure on Russia must be increased, including with increased aid to Ukraine," Wadephul said.

The talks on Monday in Washington are about establishing "the elements of a negotiated solution on the path to a just peace for Ukraine," he said.

"Firm security guarantees are crucial for this. Because Ukraine must be able to defend itself effectively even after a ceasefire and peace agreement."