SEOUL, South Korea — China expressed concern about the possible deployment of an advanced US missile defense system in South Korea during talks Wednesday between their defense chiefs, Seoul military officials said.

Washington is considering whether to instal the politically sensitive THAAD (Terminal High-Altitude Area Defense) system in the South, a close US ally which hosts 29,000 American troops.

US officials have tried to portray any THAAD system based in South Korea as non-threatening. It is designed to shoot down ballistic missiles at a higher altitude with a "hit-to-kill" approach.

At talks in Seoul with his South Korean counterpart Han Min-Koo, Chinese Defence Minister Chang Wanquan expressed "concern" at the US move, Han's office said.

In response, it said, Han clarified South Korea's position that there have been no formal discussions about the THAAD deployment. The office declined to disclose details.

South Korea has been reluctant to take part in a proposed US-led regional missile defense system because China and Russia view it as a threat to their security.

It was the first time that a ranking Chinese official had publicly raised the THAAD issue with South Korea, according to Yonhap news agency.

The ministers agreed to establish a hotline between them as soon as possible, Yonhap said, adding related talks would probably begin next week.

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Last year South Korea and China signed a memorandum of understanding to set up the hotline, but it has yet to be finalized.

The two have been operating telephone hotlines between their navies and air forces since 2008 to help prevent accidental clashes.

China, despite being North Korea's sole major ally, has gradually strengthened military cooperation and exchanges with South Korea.