Last week, China's coast guard mounted what the Philippines called an "unusual" show of force at Second Thomas Shoal, a hotly disputed submerged reef in the Spratly Islands where Manila maintains a small outpost in the form of the rusting warship BRP Sierra Madre, grounded there in 1999 to anchor its territorial claim.

President Donald Trump told reporters on Monday that China has to give the United States magnets or "we have to charge them 200% tariff or something" amid a trade dispute between the two nations. China is increasingly sensitive about rare earths and its control over supply,