EU warns 150,000 Russian troops massed around Ukraine
"It is the highest Russian military deployment at Ukrainian borders ever," Borrell told journalists after talks involving Ukraine's foreign minister, refusing to disclose the source of the figure.
"The risk of further escalation, it's evident," he warned.
Tensions have spiralled over the build-up of Russian troops around its southwestern neighbour as an intensification of clashes with Moscow-backed separatists has fuelled fears of a return to widespread fighting in Ukraine.
Moscow's military says it is conducting exercises along its frontier in response to moves by Western military alliance NATO that "threaten Russia".
Defence Minister Sergei Shoigu said last week that "two armies and three airborne units were successfully deployed" to the Russia's western border and that the drills would be over "within two weeks".
Ukraine is pushing the West for more practical support as it looks to deter any further aggression from Moscow.
But Borrell said that there were currently no further sanctions being proposed or under consideration.
The Russian build-up comes as clashes between Ukrainian forces and Moscow-backed separatists have flared in the east in recent weeks, shredding a ceasefire brokered last year.
A Ukrainian soldier was killed and one more wounded in the latest bloodshed in the conflict-ridden east, the military said Monday.
Kiev has been battling pro-Russian separatists in the eastern Donetsk and Lugansk regions since 2014, following Moscow's seizure of Crimea.