South Korea on Thursday criticized the trip of a top Japanese aide to Pyongyang as “unhelpful” amid tensions with the isolated regime following weeks of warlike rhetoric.

Foreign Ministry Cho Tai-Young said the visit made by Isao Iijima, Japanese Prime Minister Shinzo Abe’s close adviser, would only undermine a united front against North Korea.

“In that sense, we think that the visit by Iijima to North Korea is unhelpful,” he said.

Washington said the trip had also taken them by surprise. “I think obviously we look forward to hearing from the government of Japan more details about this in coming days,” said Glyn Davies, the US special representative on North Korea, who is in Tokyo on the third leg of an East Asia tour.

The visit has raised speculations of a thaw in relations between North Korea and Japan.

China expressed support for the trip, saying that Beijing hopes “a relevant engagement can help relax the tension on the Korean peninsula and maintain peace and stability of this region,” according to Chinese foreign ministry spokesman Hong Lei.