As interest rates turn negative around the world, the Federal Reserve is asking banks to consider the possibility of the same happening in the U.S.

In its annual stress test for 2016, the Fed said it will assess the resilience of big banks to a number of possible situations, including one where the rate on the three-month U.S. Treasury bill stays below zero for a prolonged period.

"The severely adverse scenario is characterized by a severe global recession, accompanied by a period of heightened corporate financial stress and negative yields for short-term U.S. Treasury securities," the central bank said in announcing the stress tests last week.

In that particular simulation, the unemployment rate doubles to 10 percent, the same level it reached in the aftermath of the last financial crisis.

Three-month bill rates have slipped slightly below zero several times in recent years, including in September after the Fed delayed rate liftoff amid global financial market turmoil, touching a low of minus 0.05 percent on Oct. 2.

But in the stress test, banks would have to handle three- month bill rates entering negative territory in the second quarter of 2016, and then falling to negative 0.5 percent and holding there through the first quarter of 2019.

Not a Forecast

"This scenario does not represent a forecast of the Federal Reserve," the central bank said. It also assumes "that the adjustment to negative short-term rates proceeds with no additional financial market disruptions."

Fed officials have made clear that they are a long way from contemplating a reduction in rates below zero in their benchmark overnight policy rate. Some, though, have suggested they’d be more open to such a move than in the past should the economy deteriorate significantly.

The central bank left its target range for the federal funds rate unchanged at 0.25 percent to 0.5 percent last week after raising it in December for the first time since 2006.