The United States should exercise "outside pressure" on Japan to get the ally to put aside its complaint over a South Korean statue symbolizing Tokyo's wartime sexual slavery and work hard to mend ties with Seoul, a former State Department official said.

Mintaro Oba, a former diplomat handling Korea-Japan relations, made the remark in an article to The Diplomat, strongly criticizing Tokyo for overreacting to the establishment by civic activists of the girl's statue in front of Japan's consulate in the South Korean port city of Busan.

Japan has demanded the statue be removed, calling it a violation of the 2015 agreement that the two countries reached on resolving the sexual slavery issue. In protest, Tokyo has also recalled its ambassador and consul general from the South and put currency swap talks on hold.

"Japan's high-level response to an action by a civic group outside of Seoul's control makes a mountain out of a mole hill.

It's an appalling error in strategic judgment that will endanger Korea-Japan relations at a time when unity among US allies is critical to deterring regional aggression," Oba said.

"The United States must steer Japan toward a stance that reduces tensions and advances Korea-Japan relations," he said.

He also said that the next US ambassador to Japan should actively make the case to Japan's ruling party leaders that further historical reconciliation serves Japan's security interests.

Effective "outside pressure" from the US is critical to move the two key allies toward reconciliation, he said.

Japan could have limited its response to diplomatic channels or lodged a public protest, rather than recalling its ambassador and cutting cooperation, Oba said. Such overreaction stoked "Korean doubts about Japan's sincerity in addressing its wartime wrongs and empowers critics of Japan," he said.

The renewed tensions came at a bad time when the Korea-Japan ties have become more vulnerable as uncertainty has grown for the fate of the 2005 agreement in the wake of the impeachment of South Korean President Park Geun-hye, Oba said.

"Japan is an easy mark in Korean politics, and the comfort women agreement's association with the unpopular Park makes it even more likely to become a campaign issue. Leading candidate Moon Jae-in has already called for renegotiation of the agreement," he said.