China and Russia are working on a joint invasion of Taiwan, US intelligence officials fear.

'We see China and Russia, for the first time, exercising together in relation to Taiwan and recognising that this is a place where China definitely wants Russia to be working with them, and we see no reason why they wouldn't,' Avril Haines, Director of US National Intelligence, told the US Congress on Thursday.

Her warning on the potential cooperation between the countries on Taiwan, which China has long claimed as its own territory to be annexed by force if necessary, was echoed by Lieutenant General Jeffrey Kruse.

He said that the US Defence Department has 'become even more concerned about our joint force requirements in an environment where [Russia and China] would certainly be cooperative, and we need to take that into account,' Bloomberg reports.

A potential joint invasion of Taiwan was raised in the meeting of the Senate Armed Services Committee by Republican Senator Mike Rounds, who asked the head of the Defence Intelligence Agency about the Pentagon's plans should an invasion happen. 

Russian President Vladimir Putin shakes hands with Chinese President Xi Jinping during a meeting at the Belt and Road Forum in Beijing, China, October 18, 2023

Avril Haines, right, director of National Intelligence, speaks as Lt. Gen. Jeffrey Kruse, left, director of the Defense Intelligence Agency, listens during the open portion of a hearing of the Senate Armed Services Committee on Capitol Hill, Thursday, May 2, 2024

Haines said that there seems to be an 'increasing cooperation in the "no-limits" partnership' between Russia and China.

This comes as Chinese leader Xi Jinping is expected to make his first trip to Europe in five years next week as part of China's efforts to rebuild its foreign relations after a prolonged absence during the Covid-19 pandemic.

READ MORE: China buys huge £135billion stockpile of gold raising fears Beijing is preparing to invade Taiwan and protect its economy in the event of Western sanction 

Xi will start the tour in Paris on Monday, meeting with French President Emmanuel Macron, who has been stressing the idea of European strategic autonomy from the US.

On a visit to Beijing last year, Macron courted controversy by saying France would not necessarily always align with the US in foreign policy, an apparent reference to American support for the self-governing republic of Taiwan, which China claims as its own territory to be annexed by force if necessary.

After leaving France, Xi will visit Hungary and Serbia, both seen as China-friendly and close to Russian President Vladimir Putin, rebuffing Western criticism of his full-scale invasion of Ukraine.

Xi's European visits will be closely followed in Washington for signs of diminishing support for its key foreign policy goals.

The Chinese leader will arrive in France just as Paris is putting the finishing touches on its preparations for hosting the Summer Olympics, an event in which China invests huge amount of national prestige.

France sees Xi's visit, which officially marks 60 years of French-Chinese diplomatic relations, as an important diplomatic moment, and wants to focus on China's broader relations with the EU. 

Macron invited European Commission President Ursula von der Leyen to the talks Monday.