DOHA, Qatar — A separatist group has seized control of an oil-rich region in southern Yemen, shattering a relative calm in the country’s stalemated civil war — a significant move in a country located along a key international trade route that also threatens to bring new risks to the Persian Gulf region.

The secessionist Southern Transitional Council, STC, a group backed by the United Arab Emirates, this month seized most of the the provinces of Hadramout and Mahra, including oil facilities.

Yemen has been mired for more than a decade in a civil war that involves a complex interplay of sectarian grievances and the involvement of regional powers.

The Iran-aligned Houthis control the most populous regions of the country, including the capital Sanaa. Meanwhile, a loose regional coalition of

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