China on Tuesday scolded South Korean and Japanese lawmakers for visiting Taiwan despite its strong opposition, chiding both neighbors for attending Taiwan's "so-called inauguration ceremony of the leader".
Taiwan's President Lai Ching-te, regarded by China as a "separatist", was officially sworn in on Monday after winning a January election.
China, claiming Taiwan as part of its territory, resolutely opposes the visits, its embassies in South Korea and Japan said, and has lodged solemn representations with the two northeast Asian countries.
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A spokesperson for the embassy in South Korea said the move runs counter to the China-South Korea strategic cooperative partnership, while to Japan, the embassy there said the visit seriously contravened the spirit of Sino-Japanese commitments.
China urged South Korea to take "practical actions" to safeguard overall interests of bilateral relations while it seriously urged Japan to cease "provocative political manipulation" of the Taiwan issue.
It also told Japan, which has joined the United States in congratulating Lai, to avoid causing further damage to peace and stability in the Taiwan Strait and to their bilateral relations.
On Monday, Japan's Chief Cabinet Secretary Yoshimasa Hayashi called Taiwan "an extremely important partner and friend of our country", and that Japan's stance was to further deepen cooperation and exchanges at the non-governmental level.
Ties between Japan and China have sagged in recent times, weighed down by a host of issues including clashes over mutual maritime claims in the East China Sea, as well as China's actions against Philippine claims in the South China Sea.
China, South Korea and Japan are due a trilateral summit next week in Seoul.
Last week, Chinese Foreign Minister Wang Yi told his South Korean counterpart who visited Beijing that both nations should seek stable ties despite their recent "difficulties" including tensions over Taiwan and other regional issues.
South Korea's Cho Tae-yul told Wang both countries should work together and "even if there are difficulties, momentum of cooperation should be continued while carefully managing the relations".
South Korean President Yoon Suk Yeol said last year that democratically governed Taiwan, which China claims as part of its territory, was a "global issue", not just an issue between China and Taiwan.
"Anyone who attempts to challenge the one-China principle will end in failure," Wang warned on Monday as Lai was sworn in.
"There is only one China in the world, and Taiwan is part of China."