Europe is braving its most serious security threat since the Cold War, the European Union's foreign policy chief Josep Borrell warned Monday, while still voicing hope for a diplomatic resolution to the standoff with Russia over Ukraine.

Questioned over United States warnings of an imminent Russian invasion of Ukraine at a joint news conference in Washington with Secretary of State Antony Blinken, Borrell said they shared "a strong concern" about the situation on the ex-Soviet state's borders.

"We are living, to my understanding, the most dangerous moment for security in Europe after the end of the Cold War," Borrell told reporters.

"Nobody masses 140,000 soldiers heavily armed in the border of a country" without it representing "a strong threat," he said.

Borrell underlined that the 140,000 troops massed by the border are not there "to have tea."

U.S. officials say Moscow has assembled 110,000 troops near the border with Ukraine and is on track to amass a large enough force – some 150,000 soldiers – for a full-scale invasion within weeks.

Blinken denied that Washington's stance was alarmist, saying: "This is not alarmism. This is simply the facts."

Both the United States and European Union are threatening to retaliate with unprecedented economic sanctions should Russian President Vladimir Putin move ahead with an invasion of neighboring Ukraine.

"We don't believe that Mr President Putin has made a decision, but he has put in place the capacity, should he so decide, to act very quickly against Ukraine, and in ways that would have terrible consequences for Ukraine, for Russia, but consequences also for all of us," said Blinken.

Nevertheless, both Blinken and Borrell stressed that diplomacy was still hard at work to bring the standoff to a peaceful resolution.

"We believe that a diplomatic way out of the crisis is still possible," summed up Borrell. "We hope for the best, but we prepare for the worst."