BRUSSELS (BLOOMBERG) - China's efforts to tighten its grip on Hong Kong pose a threat to the rules-based international order, the European Union's top diplomat said, calling on member states to respond with a "robust" message.

China's increasing control over the city "affects not only our direct interests in Hong Kong but also the maintenance of the rules-based international order where legal and political commitments are to be respected," EU foreign policy chief Josep Borrell said in a letter to the bloc's 27 foreign ministers.

The EU must continue "to ensure unified and robust messaging," Mr Borrell said in the letter dated May 26 and seen by Bloomberg.

EU's top diplomats will hold a video call Friday (May 29) to discuss relations with China amid increasing unease in the region over Beijing's moves in Hong Kong and what is seen as an effort to capitalise on the pandemic to boost its global status.

Effectively the world's third biggest economy, the EU has been under intense pressure to pick sides in the spat between its two biggest trading partners - the US and China - a balancing act that risks alienate both.

"Beijing's rise to be an assertive, capable and self-confident global actor will be a test to the EU's geopolitical ambitions," Mr Borrell said in his letter setting the stage for Friday's discussion.

The bloc must decide "how to ensure unity among member states and coherence between EU policies, in the face of China's increasing assertiveness and attempts to influence and shape global public opinion and perceptions as part of its wider geopolitical strategy."

While sharing many of US President Donald Trump's concerns about unfair trade practices and the risk of espionage, the EU has resisted US pressure to attack Beijing with blanket tariffs, or ban Chinese companies from the region's telecom networks.

Instead, Brussels has been pushing for an opening of the Chinese economy to foreign firms while seeking to maintain communication channels, combining anti-dumping duties on Chinese imports with ad hoc coordination in areas such as climate change and Middle East policy.

The Hong Kong crackdown could put that strategy to the test.

The US is considering a range of sanctions and the EU may be forced to follow suit if violence escalates.

"We will certainly have to reflect on the impact of these moves on EU-China relations," Mr Borrell said in his letter, even though the prospect of punitive measures isn't formally on the agenda for Friday's meeting of foreign policy chiefs.