BEIJING (Yonhap) ― Chinese President Xi Jinping asked South Korean President Park Geun-hye to “tread carefully” over a U.S. move to deploy its advanced missile-defense system in South Korea during their July summit, two Seoul diplomatic sources said Tuesday.

Xi had raised the issue during his summit with Park in early July at a time Beijing was trying to improve relations with South Korea amid bitter territorial disputes with its Asian neighbors, including Japan, Vietnam and the Philippines. South Korea is a treaty ally of the U.S., which protected the South from aggression by North Korea and China during the 1950-53 Korean War.

South Korea is home to about 28,500 American troops and the top U.S. commander in South Korea, Gen. Curtis Scaparrotti, said in early June that Washington wants to deploy the THAAD (Theater High Altitude Area Defense) missiles to South Korea to better defend against missile threats from North Korea.

China and Russia have voiced concerns over the U.S. move to deploy the new missile-defense shield to the southern part of the Korean Peninsula.

Former South Korean defense minister Kim Kwan-jin, who now serves as the top national security adviser to Park, told lawmakers on June 18 that it would be “no matter” if the U.S. troops stationed in South Korea deploy the THAAD to the country.

“During the summit talks, President Xi told President Park that it needs to tread carefully over the issue of the THAAD deployment (to South Korea),” a diplomatic source who was briefed about the July summit told Yonhap News Agency.

It was not known how Park responded to Xi’s call.

Another diplomatic source said Park and Xi discussed the issue “in a specific manner” during the summit, but declined to elaborate on specifics of the discussion.

In spite of international sanctions, North Korea continues to pose security threats to Northeast Asia and beyond, by developing its nuclear and ballistic missile programs.

North Korea, which has conducted three nuclear tests since 2006, has threatened to conduct a “new form of nuclear test.”

South Korea and the U.S. have called on China to play a greater role in leading North Korea to give up its nuclear weapons, but Beijing‘s diplomacy still appears to put its priority on stability, rather than the denuclearization of North Korea.