Federal immigration officers may deport immigrants with as little as six hours’ notice to countries other than their own even if officials have not provided any assurances that the new arrivals will be safe from persecution or torture, a top official said in a memo this week.

Todd M. Lyons, the acting director of U.S. Immigration and Customs Enforcement, wrote in a memo to the ICE workforce Wednesday that a Supreme Court ruling last month had cleared the way for officers to “immediately” start sending immigrants to “alternative” countries.