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.