As Donald Trump and Xi Jinping met in China and agreed the Strait of Hormuz must stay open, Iran, one of Beijing's closest allies, is causing fresh chaos in the vital passageway.

A commercial ship passing through the Strait of Hormuz off the United Arab Emirates was hijacked on Thursday and is now being steered toward Iranian waters, a British maritime agency says.

The vessel was boarded by 'unauthorized personnel' while anchored 38 nautical miles northeast of the UAE, the United Kingdom Maritime Trade Operations centre said. The ship and its captors have not been identified.

The Iranian regime, meanwhile, has bragged that they allowed over 30 vessels to safely pass through Hormuz with 'approval from Tehran', according to state media. 

During their monumental diplomatic summit, Trump and Xi reportidely agreed that the Strait of Hormuz, which transports one-fifth of all global oil, must be reopened. 

'The two sides agreed that the Strait of Hormuz must remain open to support the free flow of energy,' said a White House official. 

Xi reportedly made clear China opposes any militarization of the Strait or attempts to impose tolls on its use, and signaled interest in buying more American oil to cut China's reliance on the waterway over time. 

'Both countries agreed that Iran can never have a nuclear weapon,' the White House official added.