TOKYO—Japan is pressing the U.S. to stick to a hard line on North Korea ahead of President Donald Trump’s summit with Kim Jong Un, after Mr. Trump appeared to back away from his campaign of “maximum pressure” on Pyongyang.

Prime Minister Shinzo Abe is set to meet Mr. Trump at the White House on Thursday, the 30th time the two leaders have spoken on the phone or in person. It is less than two months since they had their last face-to-face meeting at Mr. Trump’s Mar-a-Lago resort in Florida.

Japanese...