The South Korean opposition’s escalation of combative rhetoric on Japan’s plan to release treated wastewater from the crippled Fukushima nuclear power plant is “unseemly,” the ruling party's floor leader said Wednesday, Eastern European Time.

“My colleagues across the aisle have chosen to abandon the longstanding courtesy of stepping back from domestic political attacks on the president while he is on a trip meeting foreign leaders,” Rep. Yun Jae-ok of the People Power Party said in a lunch meeting with reporters, held hours before President Yoon Suk Yeol was set to meet his Japanese counterpart, Fumio Kishida, for a summit in Lithuania.

The summit between the leaders of South Korea and Japan was set to be held on the sidelines of a NATO summit in Vilnius later on Wednesday afternoon, amid fierce protests from the Korean president’s rival party at home over the Japanese government’s plan to discharge the wastewater from the Fukushima Daiichi Nuclear Power Plant into the sea.

Yun said it was a “shame” that the main opposition party was continuing to engage in politicization of diplomatic affairs while the president is away.

“Back in the day, the custom was not to let the president get distracted by domestic political disputes ahead of summit meetings, and at crucial and sensitive diplomatic junctures,” he said. “It’s regrettable that is no longer the case.”

The Democratic Party of Korea has criticized Yoon as being “soft” on Japan and its plan to discharge into the sea of some 1.33 million metric tons of accumulated wastewater used to cool three of the nuclear power plant’s reactors that melted down during the magnitude 9.0 earthquake and tsunami in March 2011. The Japanese government has said the water will be treated and diluted to safe levels, a plan that was okayed by the International Atomic Energy Agency.

For the second time this year, a delegation of Democratic Party lawmakers are on a trip to Japan to rally against the contentious plan, questioning its safety.

In Wednesday’s press conference in Tokyo, lawmakers from the Korean opposition party slammed the Japanese government as “attempting an illegal act of nuclear terrorism against the entire human race.” The discharge of the Fukushima wastewater was a “clear violation” of the United Nations Convention on the Law of the Sea, they said, calling on Japan to discard its plan.

The opposition lawmakers also took issue with the neutrality of the IAEA’s report concluding the release plan to be in line with safety standards. They accused the intergovernmental watchdog under the UN system of having a pro-Japan bias and of being motivated by intentions of promoting the atomic energy industry.

The IAEA report was “no excuse” for Japan to follow through with the plan, which could affect the lives and health of people all over the world, they said.

Democratic Party lawmakers met with the director general of the IAEA, Rafael Grossi, on Sunday in Seoul and confronted him about the report giving Japan the green light to discharge the water. At the meeting, the lawmakers urged the UN agency chief to ask Japan put its wastewater discharge plan on hold.