China downgraded its diplomatic ties with Lithuania on Sunday, expressing strong dissatisfaction with the Baltic State after Taiwan opened a de facto embassy there, escalating a row that has sucked in Washington.

China views self-ruled and democratically governed Taiwan as its territory with no right to the trappings of a state and has stepped up pressure on countries to downgrade or sever their relations with the island, even non-official ones.

Lithuania expressed regret over China's move but defended its right to expand cooperation with Taiwan, while respecting Beijing's "One China" policy, and said its foreign minister would go to Washington to discuss trade and investment projects.

Taiwan, meanwhile, reported that two Chinese nuclear-capable...