The "highest military deployment of the Russian army on the Ukrainian borders ever" is under way, according to Josep Borrell, high representative of the European Commission.

He said 150,000 troops are stationed in Crimea and along the eastern border of Ukraine, as well as field hospitals, which would be required for a sustained military campaign. 

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After a meeting of EU foreign ministers on Monday, Borrell said they "have to commend Ukraine for its restrained response and we urge Russia to de-escalate and to defuse tensions ... as the risk of further escalation is evident."

He reaffirmed "strong support for Ukraine's sovereignty and territorial integrity," but admitted the EU would not be imposing any further sanctions on Russia for now.

Dmytro Kuleba, Ukraine's foreign minister, joined the 27 EU foreign ministers on the video call.

He has spent the past few weeks lobbying European leaders for more support but has yet to receive any specific assurances or promises of new military assets.

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The U.S. halted a pre-planned deployment of warships to the Black Sea, which borders both Russia and Ukraine, in a bid not to escalate tensions.

But the UK is planning to send two warships next month.

Borrell said that, as well as Ukraine, the EU also stands united with Czechia, which expelled 18 Russian diplomats at the weekend after concluding Russian agents were responsible for an explosion at an ammunition depot on Czech soil in 2014, killing two people.

Prime Minister Andrej Babis says he believes the original plan was to blow up the ammunition only after it was transported to Bulgaria but it was detonated by mistake in Czechia.

The National Center against Organized Crime announced it was looking for two men in connection with that incident – they are the same men suspected of people suspected of the poisoning of former Russian agent Sergei Skripal and his daughter Julia in Salisbury, UK, in 2018.

For that, the UK expelled Russian diplomats and successfully encouraged other European allies to do the same.

But Borrell confirmed Czechia had not asked other European allies to also expel diplomats on this occasion.

Russia denies any involvement in either incident.

Borrell confirmed the European Commission will attend the Crimean Platform Summit in Ukraine on August 23, after the cabinet chief of its president, Ursula von der Leyen, turned down an invitation to the event on her behalf, in violation of diplomatic protocol.