The United States will put more pressure on South Korea to improve ties with Japan amid budding signs of a thaw in their relations, a U.S. congressional source said Tuesday.

The U.S. has sought to strengthen trilateral ties with its two Asian allies in part to keep a rising China in check. It has often urged the two countries to put their shared history behind and look toward the future, drawing angry reactions among South Koreans who view the U.S. as siding with Japan.

Relations between South Korea and Japan have long been marred by disputes over history and territorial issues stemming from Japan's 1910-45 colonial rule of the Korean Peninsula.

"The problem for South Korea-U.S. relations, as we saw from Prime Minister Abe's visit in April, the U.S. is fully on board with Abe's agenda," the source said during a visit to Seoul, citing the TPP and Abe's push to expand the scope of Japanese military action.

"As long as Prime Minister Abe avoids taking flagrant actions or words on history, pressure from the U.S. is mostly going to be on the Blue House," the source added, referring to South Korea's presidential office.

"The bottom line is, U.S.-South Korea tension over Japan will continue over the next year. As the U.S.-Japan partnership continues to evolve, this will become more problematic." (Yonhap)