US Republican presidential candidate Donald Trump is ready to act "immediately" as a peace broker in the Russia-Ukraine war if he is elected in November, Hungarian Prime Minister Viktor Orban has said in a letter to European Union (EU) leaders.

The letter, addressed to European Council president Charles Michel and shared with all EU leaders, was drafted after Orban held talks with Trump as well as with the leaders of Ukraine, Russia and China.

"I can [...] surely state that shortly after his election victory, he will not wait until his inauguration, [Trump] will be ready to act as a peace broker immediately. He has detailed and well-founded plans for this," Orban wrote.

Nationalist leader Orban, a long-time Trump supporter, made the surprise visits to the capitals in the past two weeks on a self-styled "peace mission" after Hungary took over the rotating presidency of the EU.

EU leaders denounced his actions, stressing Orban had no mandate to speak on behalf of the 27-nation EU and whatever views he presented were his own.

To underline the displeasure with Hungary's rogue diplomacy on the war in Ukraine, which undermines long-held EU positions, the European Commission on Monday took the unprecedented step of barring EU Commissioners from attending meetings held in Hungary under the country's presidency.

Some EU governments also privately plan to send only top civil servants, rather than government ministers, to ministerial talks in Hungary and 63 European Parliament lawmakers have asked the EU to withdraw Hungary's voting rights in the bloc.

In the letter to EU leaders, Orban said that US President Joe Biden was "making immense efforts" to remain in the electoral race, and suggested he was "not capable of modifying the current US pro-war policy."

Orban has long criticised European military support for Ukraine in contrast to most allies who support Kyiv's war effort.

The Hungarian leader said a Trump victory would change the burden between the US and the EU when it comes to financial support for Ukraine, to the disadvantage of the Europeans.

"Our European strategy in the name of transatlantic unity has copied the pro-war policy of the US. We have not had a sovereign and independent European strategy or political action plan up to now," he said.

"I propose discussing whether the continuation of this policy is rational in the future."

Orban suggested "reopening direct lines of diplomatic communication with Russia" while maintaining high-level contacts with Kyiv as well as conducting talks with China "on the modalities of the next peace conference".

Orban's approach clashes with the conclusions of the last EU summit on June 27, when leaders reconfirmed their unwavering commitment to provide continued political, financial, economic, humanitarian, military and diplomatic support to Ukraine and "as long as it takes and as intensely as needed".

  - Reuters