The European Union believes only "a spark" could start a confrontation near Ukraine's border, where it estimates 100,000 Russian troops have gathered.
EU foreign policy chief Josep Borrell said: "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. Well, a spark can jump here or there."
Initially, he told reporters there were more than 150,000 Russian troops amassing on the Ukrainian border and in Crimea, before officials corrected it in the transcript, saying the real figure was more than 100,000.
Nevertheless, Mr Borrell said the risk of further escalation is "evident".
He declined to say where he got the initial 150,000 number from, but called it "my reference figure".
It was higher than the 110,000 estimate provided by Ukrainian defence minister Andriy Taran on Wednesday.
Mr Borrell said: "All in all, the relations with Russia are not improving, but the contrary, the tension is increasing in different fronts.
"We call on Russia to withdraw their troops."
At the weekend, French President Emmanuel Macron said that while talks with Russia are crucial, "clear red lines" carrying possible sanctions must also be drawn with Moscow over Ukraine.
Mr Borrell also said that the condition of jailed Russian opposition leader Alexei Navalny is "critical" and EU member states would hold the Kremlin accountable for his wellbeing.
However, Mr Borrell outlined, following a virtual meeting of EU foreign ministers, "for the time being, there is no move in the field of more sanctions" to place on Moscow.
He also said there is no request for a synchronised EU move of expulsions in the standoff between member state the Czech Republic and Russia, following accusations from Prague that Moscow played a role in an ammunition depot explosion in 2014.
Meanwhile, Foreign Secretary Dominic Raab has said the UK "stands in full support of our Czech allies" after authorities in the country said they were hunting two men whose passports match the names of the suspects in the 2018 Salisbury Novichok attack.
On Saturday, Czech police published photos of two foreign citizens who visited the country in 2014 and asked the public for any information about them.