SEOUL, South Korea — South Korea said on Wednesday that it would press ahead with its plan to open a diplomatic “liaison office” in North Korea this year, dismissing concerns that it was too quickly making overtures to the North Koreans, who have yet to begin dismantling their nuclear weapons program.

When South Korea’s president, Moon Jae-in, held his first summit meeting with the North Korean leader, Kim Jong-un, on April 27, they agreed to establish a joint liaison office — a potential first step toward building formal diplomatic missions — in the North Korean border town of Kaesong.

In a speech last Wednesday, Mr. Moon said the opening of the office was imminent. “In a few days,” he said, “an era in which the two Koreas communicate with each other around the clock will commence.”

But the conservative political opposition and the conservative news media in the South have since raised fears that Mr. Moon’s government is creating a rift with Washington by advancing inter-Korean relations when North Korea has taken no clear steps toward denuclearization. They also questioned whether fuel oil and electricity, which the South plans to supply the office, would violate United Nations sanctions.