Josep Borrell said Russia's 100,000 troops massed at the border with Ukraine is its highest ever military deployment at that border, and warned: "When you deploy a lot of troops, a spark can jump here or there."
Russia has deployed 100,000 troops at the border with Ukraine, its biggest-ever military deployment there, the EU's foreign affairs chief has said.
Josep Borrell spoke in Brussels following a virtual meeting of EU foreign ministers, where he gave an update on a variety of issues, including Russia's proxy war with Ukraine, and the treatment of Putin critic Alexei Navalny.
Speaking at a news conference Borrell said Russia was deploying "all kind of materials" on the border including "campaign hospitals" and "warfare".
He initially said the number of Russian troops stood at 150,000. Asked to elaborate on where that figure came from, he said he couldn't but that it was his "reference figure". His services later corrected the figure to over 100,000.
"It’s the highest military deployment of Russian army in Ukrainian borders ever. It’s clear it’s a matter of concern," he said, warning: "When you deploy a lot of troops, a spark can jump here or there."
Relations between Russia and Ukraine have continued to worsen, after Russia expelled a Ukrainian diplomat it accused of receiving classified information.
Ukraine expelled a Russian diplomat in response, with the moves coming as tensions escalate over Russia's military buildup along Ukraine's eastern border.
Russia's defence minister has claimed the military buildup is a response to what he described as threats from NATO.
Sergei Shoigu said last week the deployment in western Russia was part of readiness drills that would continue for two weeks.
NATO chief Jens Stoltenberg last week described the deployment as "unjustified, unexplained and deeply concerning".
The US State Department, meanwhile, expressed "deep concern" on Monday "over Russia's plans to block foreign naval ships and state vessels in parts of the Black Sea, including near occupied Crimea and the Kerch Strait."
"Russia has a history of taking aggressive actions against Ukrainian vessels and impeding access to Ukraine’s ports in the Sea of Azov, impacting Ukraine’s international commerce. This represents yet another unprovoked escalation in Moscow's ongoing campaign to undermine and destabilise Ukraine," State Department spokesperson Ned Price added.
The US ambassador to Russia, John Sullivan, is to travel to Washington this week for consultations with the Biden administration.
Borrell also discussed the situation of jailed Kremlin critic Alexei Navalny and described it as "critical" He demanded that Russian authorities allow him access to medical professionals he trusts.
He said he travelled to Moscow in February, to raise the issue "face-to-face" with the authorities, but "the request was not heard".
"Today we are passing a united message to Russian authorities - they are responsible for Navalny’s health, and they will be held responsible for it."
Navalny was to be transferred to a hospital in another penal colony in Russia, the Russian penitentiary service (FSIN) announced on Monday.
His health was said to be deteriorating amid a hunger strike that has entered its third week.
Russia claims his health condition is "satisfactory".
His supporters have dismissed his transfer to a hospital for convicts in another penal colony as a ploy intended to lower turnout at pro-Navalny protests scheduled for Wednesday.
Navalny's team have raised the alarm about his state of health, who they say has lost 50kg in less than three weeks, and is in danger of going into cardiac arrest.