SEOUL, Aug. 11 -- Lawmakers of three South Korean opposition parties held their first gathering to oppose the deployment of Terminal High Altitude Area Defense (THAAD) on its soil, Yonhap news agency reported on Thursday.

Members of three political parties, including the main opposition Minju Party, the casting vote-exercising People's Party and the minor Justice Party, held the first closed-door meeting to discuss how to launch a tri-partisan anti-THAAD gathering.

Participating in the meeting were four Minju Party lawmakers and three People's Party members. One Justice Party member was originally set to join the meeting, but he was absent as he visited Seongju county, where one THAAD battery is set to be sited by the end of next year under the Seoul-Washington agreement.

The People's Party and the Justice Party have demanded the retraction or re-negotiation at the National Assembly of the THAAD deployment decision, which will strain relations of South Korea with China and Russia and escalate tensions in the region.

The Minju Party has been divided into those clearly against the THAAD installation and members taking cautious stance ahead of the presidential election in late next year. About half of South Koreans are against the U.S. missile shield, while the rest half is in favor of it.

The ruling Saenuri Party claims the parliamentary agreement is not required in the THAAD deployment as it is a matter of national security, but opposition lawmakers demand an open debate as it causes serious effects militarily, diplomatically and economically.

As the ruling party lost its majority in the April general elections, it cannot win approval at the National Assembly for the THAAD deployment if the parliamentary ratification process is launched.

The tri-partisan anti-THAAD gathering is expected to be formally launched in September after one more preparatory meeting among the lawmakers, according to Yonhap. About 30 lawmakers are forecast to join the anti-THAAD gathering.