WASHINGTON (AP) — The U.S. is appealing to its closest Asian allies, Japan and South Korea, to improve their strained relations, saying there’s an urgent need to show restraint on “difficult historical issues.”

Assistant Secretary of State Daniel Russel’s comments Tuesday reflect growing angst in Washington over the widening gulf between Tokyo and Seoul over Japan’s attitude to its wartime and imperial past.

Deepening that standoff, Japan recently announced it is reviewing evidence on which a landmark 1993 apology over forced prostitution during World War II was based. South Korea has warned that would only isolate Japan.

In congressional testimony, Russel didn’t apportion blame for the strained relations. He said dedicated efforts by both parties are needed to calm tensions, but added, historical issues should be handled “in a way that promotes healing.”