President Donald Trump says "we'll have to see" if his planned summit next month with North Korean leader Kim Jong Un takes place after Pyongyang expressed sharp reservations about U.S. demands that it dismantle its nuclear weapons program."We haven't been notified at all. We haven't seen anything. We haven't heard anything. We'll see what happens. Time will tell," Trump said Wednesday at the White House. Trump said U.S. will still insist on denuclearization by North Korea.Earlier, White House press secretary Sarah Sanders told Fox News that Trump is prepared for "tough negotiations," but that if the June 12 summit in Singapore is called off, "we'll continue the maximum pressure campaign" of economic sanctions against North Korea. The fate of the summit was thrown into doubt after North Korea first Vice Foreign Minister Kim Kye Gwan said Pyongyang will have to "reconsider" whether to take part in the summit if the United States continues to demand that the regime unilaterally abandon its nuclear weapons arsenal.Kim said North Korea will not be interested in the Trump-Kim summit if Washington tries "to push us unilaterally into a corner and force us to give up nukes."The North Korean statement was the second in 24 hours that appeared to erode a period of improved relations between North Korea, South Korea and the United States. North Korea abruptly canceled high-level talks scheduled for Wednesday with the South, citing the current joint U.S.-South Korean military exercises. The North Korean news agency KCNA said the Max Thunder exercises are a "rehearsal for invasion" of North Korea and a provocation.The statement also warned that the U.S. "will also have to undertake careful deliberations about the fate of the planned North Korea-U.S. summit in light of this provocative military ruckus jointly conducted with the South Korean authorities."The Pentagon describes the Max Thunder exercises as a routine annual training to "enhance the ROK-U.S. alliance's ability to defend the ROK [South Korea] and enhance interoperability and readiness." It says this has been clear for many decades. Democratic Senator Chris Coons told VOA that 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," he said.Republican Senator Thom Tillis said the recent actions are part of a negotiating ploy by North Korea. "I actually think it's par for the course at this stage of any negotiations. The question is whether or not the summit stays on," he told VOA.State Department spokeswoman Heather Nauert pointed out that Kim has said he understands and appreciates the importance of such exercises to the United States. She said as of now, the U.S. is still going ahead with plans for the summit with Kim.But others who have worked closely with the North over the years say there are hardliners who may want to sabotage diplomatic negotiations they believe could imperil the Kim dynasty.Seoul said Wednesday's talks between the North and South were to have focused on demilitarization and plans to formally end the Korean War that occurred in the early 1950s.VOA's Steve Herman and Michael Bowman contributed to this report.