New Delhi, Feb. 19 -- Donald Trump has reopened a diplomatic fault line between Washington and London, urging Britain not to proceed with its plan to hand sovereignty of the Chagos Islands to Mauritius and lease back Diego Garcia, the strategically vital military base used by US and British forces.

In a sharply worded social media post on Wednesday, the US president warned that leasing arrangements were "no good when it comes to Countries" and suggested the agreement could weaken Britain's control over a key Indo-Pacific outpost at a moment of heightened tensions with Iran.

The intervention comes despite the US State Department having formally endorsed the UK-Mauritius agreement only a day earlier, underscoring the erratic messaging tha...