U.S. lawmakers of both parties have told VOA they are not surprised by North Korea's threat to cancel an upcoming summit between President Donald Trump and Kim Jong Un.Pyongyang's objections to joint U.S.-South Korean military drills and the Trump administration's stance on North Korea's nuclear program are an obvious negotiating ploy, according to Republican Senator Thom Tillis."I actually think it's par for the course at this stage of any negotiations," he said. "The question is whether or not the summit stays on." White House officials have downplayed Pyongyang's threat, and Tillis is urging a continued measured response."Nobody could have ever predicted a couple weeks ago that we would even get to the point to even schedule a [Trump-Kim] meeting. So I think we made positive progress, but we need to measure our rhetoric and try to keep it on track," Tillis said.Democratic Senator Chris Coons says Pyongyang's actions underscore the challenges of dealing with Kim."It's a reminder that he's a very wily, capable adversary, that he will be unpredictable and difficult to negotiate with, as his father and grandfather were before him," Coons said.Ben Cardin, the senior Democrat on the Senate Foreign Relations Committee, urged a continued focus on quiet diplomacy, not North Korea's often-noisy rhetoric.