BRUSSELS—The planned meeting between President Donald Trump and North Korean leader Kim Jong Un by May will make it more likely the administration will kill the Iranian nuclear agreement, some current and former western officials said.

The Kim meeting could come within days of Mr. Trump’s May 12 deadline for deciding whether to extend U.S. sanctions waivers on Iran. The president has repeatedly attacked the Iran agreement and warned he may refuse to sign the waivers, a step that would likely breach the agreement’s terms and...