China says Lithuania "shall be responsible" for the consequences that come after the opening of a de facto Taiwan embassy in the European nation.
The Taiwan Representative Office was opened on Thursday in the capital Vilnius despite the strong condemnation from China, who claims the island of 24 million as part of its territory.
In August, China re-called its ambassador to Lithuania and expelled Lithuanian diplomats from Beijing over the Baltic state's decision to open the office with the self-governing island.
A spokesperson for China's Foreign Ministry on Friday said the opening of the office "undermines China's sovereignty and territorial integrity" and called for its immediate closure.
"The Chinese government expresses strong protest over and firm objection to this extremely egregious act and will take all necessary measures to defend national sovereignty and territorial integrity," the spokesperson said.
"The Lithuanian side shall be responsible for all the ensuing consequences.
Taiwan has offices around the world, including Australia, but uses the name of the city Taipei.
Its office in Australia is called the Taipei Economic and Cultural Office while in the United Kingdom it is called the Taipei Representative Office.
The decision to use the name Taiwan in its new office in Lithuania has led to threats from Beijing over the island's attempts at independence.
"We also have this stern warning for the Taiwan authorities — seeking Taiwan independence by soliciting foreign support is a totally misguided attempt that is doomed to fail," the spokesperson said.
"There is but one China in the world, Taiwan is an inalienable part of China's territory, and the government of the People's Republic of China is the sole legal government representing the whole of China."
Chinese pressure has reduced Taiwan's formal diplomatic allies to just 15.
Taiwan's foreign ministry said the opening of the office would "charter a new and promising course" for ties between it and Lithuania.
Lithuania's Foreign Minister Gabrielius Landsbergis said the new office was part of the country's ongoing efforts to establish more relationships in the region.
Mr Landsbergis also indicated the Baltic state would consider opening its own office in Taiwan.
"We seek closer relations with the entire Asian and the Indian-Pacific Ocean region," he said.
"Lithuania opened an embassy in Australia recently, another one will open in South Korea, and also probably a representation office in Taipei in the future."
ABC/AP
