WASHINGTON ― Amid Russia’s massive troop buildup along its border with Ukraine, the top U.S. commander in Europe warned Thursday that NATO was prepared to respond to aggression.

“We deter, and if deterrence fails, we’re prepared to respond to aggression with the full weight of the trans-Atlantic alliance,” Gen. Tod Wolters said during a House Armed Services Committee hearing on the region. Wolters is U.S. European Command’s chief and NATO’s supreme allied commander for Europe.

The comments came amid heightened tension between Washington and Moscow, which has more troops on the Ukraine border than at any time since 2014. Also this week, the Biden administration hit Russia with sanctions and diplomatic expulsions in response to its SolarWinds cyberattack and ongoing occupation of Crimea, which Russia seized and then annexed from Ukraine in March 2014.

“The president is taking hard and fast action with appropriately tailored responses to provide a clear signal of our resolve without escalation,” Laura Cooper, the Pentagon’s policy lead for Russia, told the panel.

The U.S. and its allies have sounded the alarm about the concentration of Russian troops along the border with Ukraine and increasing violations of a cease-fire in eastern Ukraine, where Russian-baсked separatists and Ukrainian forces have been locked in a conflict since the annexation.

Wolters said it is “of great concern, and our vigilance is high,” as a “very, very large” ground force had moved near Crimea and Donbass, with shifts in air and maritime forces as well. He added that Russia’s efforts to move supply lines forward had, however, “plateaued.”

Wolters assessed the chance of Russia invading Ukraine as “low to medium,” adding that the “likelihood would start to wane.” He declined to publicly share a more detailed assessment.

The comments came as Russian Defense Minister Sergei Shoigu said Tuesday that the country’s massive military buildup in the west was part of readiness drills amid what he described as threats from NATO. He said they would last for another two weeks.

The White House said U.S. President Joe Biden voiced concern over the Russian buildup and “called on Russia to de-escalate tensions” during a phone call Tuesday with Russian President Vladimir Putin.

In separate meetings with Ukraine’s foreign minister, U.S. Secretary of State Antony Blinken and NATO Secretary General Jens Stoltenberg voiced strong support for Ukraine and warned Russia against pressing ahead with its troop buildup along the former Soviet republic’s eastern border.

Stoltenberg expressed NATO’s “unwavering” support for Ukraine during a news conference Tuesday with Ukraine’s foreign minister, calling the Russian movements “unjustified, unexplained and deeply concerning.”

In a hearing earlier in the week, Wolters affirmed that the Ukrainian military could repel a Russian offensive in the eastern part of the country “over time.” He said the U.S. continues efforts to train Ukrainian troops.

In the realm of security assistance, Cooper affirmed the Biden administration’s support for lethal aid to Ukraine and pointed to patrol boats and other equipment that were part of a $125 million package announced last month.

In his testimony Thursday, Wolters hailed plans this year to base new F-35 fighter jets at RAF Lakenheath in England beyond the 81 F-35s already on the continent. He also touted plans to bring two U.S. destroyers to the region by 2026 as helpful to countering Russia’s undersea activity.

The U.S. Army’s plans to build pre-positioned stocks and field its deployable airbase system in Eastern Europe should remain on track as long as Congress continues to support the special European Deterrence Initiative fund.

Amid the recent tensions, the United States notified Turkey that two U.S. warships would sail to the Black Sea on April 14 and April 15 and stay there until May 4 and May 5. The ships have made regular visits to the Black Sea in past years, vexing Moscow.

Russian Deputy Foreign Minister Sergei Ryabkov denounced the latest deployment as “openly provocative,” adding that “American ships have absolutely nothing to do near our shores.”

The Associated Press contributed to this report.

Joe Gould was the senior Pentagon reporter for Defense News, covering the intersection of national security policy, politics and the defense industry. He had previously served as Congress reporter.

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