China is using control over global seas as a tool of “political coercion,” the U.S. ambassador to the United Kingdom warned, as President Donald Trump is restating his interest in Greenland for that very same reason.

China is seeking to turn ports, ships and supply chains into instruments of geopolitical leverage, U.S. Ambassador to the U.K. Warren Stephens warned in a statement to the International Maritime Organization Council (IMO).

“I spoke about a challenge that we cannot afford to ignore: China’s systematic effort to use maritime power as an instrument of political coercion,” Stephens wrote in a Truth Social post after the speech. “Beijing has pursued an aggressive strategy of acquiring port concessions and infrastructure around the world — not simply for commercial gain, but to extend its strategic reach and weaponize that access against sovereign nations.”

The U.S. is “not a passive observer of maritime affairs,” Stephens told the IMO, calling America a “cornerstone of the global maritime economy.” He said the U.S. Maritime Transportation System supports $5.4 trillion in economic activity each year and nearly 30 million jobs.

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The warning comes as Trump continued peacemaking efforts in the Strait of Hormuz and pressing Demark and NATO for U.S. control over Greenland due to Chinese and Russian threats to free navigation in the Arctic.

“Denmark doesn’t spend money to really help Greenland, but it’s an important part for the United States, and it’s surrounded by China ships and Russian ships,” Trump said at the NATO summit in Ankara, Turkey.

For global security, Greenland “should be controlled by the United States, not by Denmark,” Trump added.

“They wouldn’t go along with it,” Trump said of Denmark. “And with all the money we spend to help them with Russia. And we don’t have to spend any money, we could remove all of our soldiers out of Europe because, as you probably noticed, Europe is a very different place than it was 20 years ago. A lot different.”

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Greenland’s position between the Arctic and North Atlantic has made it a key piece of U.S. security planning.

“President Trump has made restoring American maritime leadership a national priority,” Stephens said in his IMO remarks warning on China’s broader maritime footprint.

“The evidence is clear and growing,” Stephens said. “China currently builds more than half of the world’s ships. It dominates the production of ship-to-shore cranes and shipping containers.”

Stephens cited Panama as a recent example, pointing to a ruling by Panama’s Supreme Court that found CK Hutchison’s port concessions at the Balboa and Cristóbal terminals unconstitutional. The terminals sit at opposite ends of the Panama Canal, one of the world’s most strategically important trade corridors.

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Stephens said China’s response to the Panama ruling was “swift and punishing,” accusing Beijing of taking action against Panama-flagged vessels in a move the U.S. characterized as an attempt to undermine Panama’s sovereignty and disrupt global supply chains.

“What happened to Panama is a warning to every nation in this room,” Stephens said.

“When a country allows a foreign power or its proxies to control its ports, it does not simply accept a commercial arrangement,” he added. “It accepts a vulnerability.”

Stephens urged IMO member states to closely scrutinize deals allowing foreign entities, especially state-linked enterprises, to operate critical port infrastructure.

“Transparency, the rule of law, and genuine sovereignty are not obstacles to commerce,” Stephens said. “They are its foundation.”

The ambassador also said the U.S. would press for maritime security, sanctions enforcement, protection of flag state rights, freedom of navigation and updated standards on polar operations, autonomous vessels and cyber risk management.

“The pattern is consistent: China uses its maritime and economic power to coerce, to intimidate, and to punish those who assert their sovereign rights,” Stephens said.

The Chinese government has routinely rejected U.S. accusations that its overseas infrastructure investments are coercive, portraying its port, shipping and Belt and Road projects as commercial partnerships that support global trade and development.

Stephens said the U.S. will continue contributing expertise, resources and leadership at the IMO, but warned that the rules-based maritime order cannot be taken for granted.

“A free and open ocean is not guaranteed,” he said. “It must be defended.”