The European Union estimates that 150,000 Russian troops have already amassed for the biggest military build-up ever near Ukraine’s borders and that it will only take “a spark” to set off a confrontation.

At the same time, EU foreign policy chief Josep Borrell said the condition of imprisoned Russian opposition leader Alexei Navalny was “critical” and that the 27-nation group would hold the Kremlin accountable for his health and safety.

Despite the worrying developments, Mr Borrell said after a virtual meeting of the EU foreign ministers that “for the time being, there is no move in the field of more sanctions” to be slapped on Russia.

He also said there was not a request for a synchronised EU diplomatic move of expulsions in the standoff between EU member state the Czech Republic and Russia following Prague’s accusation that Moscow was involved in a 2014 ammunition depot explosion.

More dangerous at this time, Mr Borrell said, was the massing of Russian troops, including military field hospitals, and “all kinds of warfare”.

“It is the highest military deployment of the Russian army on the Ukrainian borders ever. It’s clear that it’s a matter of concern when you deploy a lot of troops,” Mr Borrell said.

“Well, a spark can jump here or there.”

Mr Borrell declined to say where he got the 150,000 Russian troops figure from.

But it is bigger than the 110,000 estimate provided by Ukrainian defence minister Andriy Taran on Wednesday.

More than 14,000 people have died in seven years of fighting between Ukrainian forces and Russia-backed separatists in eastern Ukraine that erupted after Russia’s 2014 annexation of Ukraine’s Crimean Peninsula.

The EU has steadfastly opposed the annexation but has been unable to do anything about it.

Efforts to reach a political settlement have stalled and violations of a shaky truce have become increasingly frequent in recent weeks across Ukraine’s eastern industrial heartland known as the Donbas.

Diplomats had expected there was little to no chance of immediate new sanctions on Moscow, but they will now seek to apply more pressure nevertheless through diplomacy.

And over the weekend, French President Emmanuel Macron said that while dialogue with Russia is essential, “clear red lines” carrying possible sanctions must also be drawn with Moscow over Ukraine.

“All in all, the relations with Russia are not improving, but the contrary, the tension is increasing in different fronts,” Mr Borrell said.

“We call on Russia to withdraw their troops,” he said.