China is set to impose new national security legislation on Hong Kong after last year’s pro-democracy unrest, a Chinese official said on Thursday, drawing a warning from President Donald Trump that Washington would react “very strongly” against the attempt to gain more control over the former British colony.

The US State Department also warned China, saying a high degree of autonomy and respect for human rights were key to preserving the territory’s special status in US law, which has helped it maintain its position as a world financial centre...