The leader of Iraq's semiautonomous Kurdish region nudged his government another step closer to full autonomy, threatening the country's fragile unity in the midst of a militant uprising.

Massoud Barzani, the president of the Kurdish Regional Government, addressing the region's parliament on Thursday, asked lawmakers to form a committee to prepare a referendum that would ask voters whether the Kurdish region should declare its statehood. The speech came as Kurdish diplomats in Washington sought to lay the groundwork for a...