The last remaining nuclear arms pact between Russia and the United States is set to expire this week, removing any caps on the two largest atomic arsenals for the first time in more than a half-century. The end of the New START Treaty sets the stage for what many fear could be an unconstrained nuclear arms race. Russian President Vladimir Putin has declared his readiness to stick to the treaty's limits for one more year if Washington follows suit, but U.S. President Donald Trump has been noncommittal about extending it. Arms control advocates have warned that the treaty's termination would fuel a renewed arms race.

President Donald Trump has declined to say whether he plans to resume underground nuclear detonation tests. He had seemed to suggest that was a possibility in a social media post this week. The post raised concerns the U.S. would begin testing nuclear weapons for the first time in three decades. The president told reporters, "You'll find out very soon" when he was asked about it. Trump said, "We're going to do some testing" and "Other countries do it. If they're going to do it, we're going to." But he refused to offer more details. He spoke aboard Air Force One as he headed to Florida on Friday for the weekend.