China has officially downgraded diplomatic ties with Lithuania, the foreign ministry in Beijing said Sunday, after Taiwan established a de facto embassy in Vilnius.

China baulks at any official use of the word "Taiwan" in case it gives international legitimacy to the island, which Beijing considers part of its territory to be taken one day.

"The Chinese government had to lower diplomatic relations between the two countries... to safeguard its sovereignty and the basic norms of international relations," the ministry said in a statement announcing the downgrade to the charge d'affaires level.

"The Lithuanian government must bear all consequences that arise from this."

It added that Lithuania had "abandoned the political commitment made upon the establishment of diplomatic relations" with China.

It was a reference to the "One China" policy, under which countries officially recognise Beijing over Taipei.

Lithuania's foreign ministry on Sunday said it regretted China's decision.

"Lithuania reaffirms its adherence to the 'One China' policy, but at the same time has the right to expand cooperation with Taiwan," the ministry said in a statement.

Lithuanian Prime Minister Ingrida Simonyte stressed that Taiwan's office in Vilnius does not have diplomatic status.

"The fact that Lithuania wants to intensify economic, cultural or scientific ties with Taiwan was announced in our government's programme, so our move should not be very surprising," she told reporters.

Chinese state broadcaster CCTV called the move a "solemn warning against the crude provocation of China's core interests".

"This fully reveals Lithuanian politicians' strategic short-sightedness," CCTV said in a commentary, adding that those who "play the so-called 'Taiwan card'" are "stupid without knowing it".

Taiwan announced in July that it would open the office, its first new diplomatic outpost in Europe in 18 years.

China has downgraded diplomatic ties with Lithuania after Taiwan established a de facto embassy in Vilnius
China has downgraded diplomatic ties with Lithuania after Taiwan established a de facto embassy in Vilnius AFP / PETRAS MALUKAS

That prompted a fierce rebuke from China. It withdrew its ambassador from Lithuania and demanded Vilnius do the same, which it eventually did.

China also halted freight trains to Lithuania and stopped issuing food export permits.

The opening of the Vilnius office is the latest sign that some Baltic and central European countries are seeking closer relations with Taiwan, even if that angers China.

In May, Lithuania announced it was quitting China's 17+1 cooperation forum with central and eastern European states, calling it "divisive".

Politicians in the Czech Republic have also pushed for closer ties with Taiwan.

Only 15 countries officially recognise Taipei over Beijing.

But Taiwan maintains embassy equivalent representative offices with many nations and several countries have similar arrangements in Taipei.

International support for the island has grown since China's President Xi Jinping came to power.

A growing number of unofficial diplomatic visits have taken place between Taiwanese, European and American officials in recent months.

Xi has ushered in a more authoritarian and muscular era, taking a markedly more aggressive approach to Taipei since the 2016 election of President Tsai Ing-wen.

She is loathed by Beijing because she regards Taiwan as an already sovereign nation and not part of "one China".

Beijing has also poached several of Taiwan's diplomatic allies in recent years, including Panama, El Salvador and the Dominican Republic.