China downgraded its diplomatic ties with Lithuania on Sunday, expressing strong dissatisfaction with Vilnius for allowing Taiwan to open a de facto embassy in the Baltic state, raising tensions in a spat that has also dragged in Washington.
China views self-ruled and democratically governed Taiwan as its own 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.
Beijing had already expressed anger that Lithuania - which has formal relations with China and not Taiwan - let Taiwan open its office in the country, and recalled its ambassador in August, reports Reuters.
The Taiwanese Representative Office in Lithuania opened on Thursday. Other Taiwan offices in Europe and the United States use the name of the city Taipei, avoiding a reference to the island itself, something that has further angered Beijing.
China's Foreign Ministry said in a brusque statement that Lithuania ignored China's "solemn stance" and the basic norms of international relations in allowing Taiwan to set up its representative office in Lithuania.
The move "undermined China's sovereignty and territorial integrity, and grossly interfered in China's internal affairs", creating a "bad precedent internationally", it said, adding relations would be downgraded to the level of charge d'affaires, a rung below ambassador.
"We urge the Lithuanian side to correct its mistakes immediately and not to underestimate the Chinese people's firm determination and staunch resolve to defend national sovereignty and territorial integrity."
No matter what Taiwan does, it cannot change the fact that it is part of China, the ministry added.
Taiwan says it is an independent country called the Republic of China, its official name, and that the People's Republic of China has never ruled it and has no right to speak for it.
Taiwan has been heartened by growing international support for it in the face of China's military and diplomatic pressure, especially from the United States and some of its allies.
Washington has offered Vilnius support to withstand Chinese pressure, and Lithuania will sign a $600 million export credit agreement with the US Export-Import Bank this week.
Only 15 countries have formal diplomatic ties with Taiwan.
Taipei could lose another ally to Beijing after the Honduran presidential election later this month, where a candidate backed by main opposition parties is leading in opinion polls.
If elected, Xiomara Castro has vowed to establish official relations with China.