On Sunday, a barrage of Russian missiles hit a large Ukrainian base near the border with NATO member Poland, killing 35 people and injuring 134, according to a local official, in an escalation of the war to the country’s west as fighting raged elsewhere.

According to Russia’s defense ministry, the air strike destroyed a large amount of weapons supplied by foreign nations and stored at the sprawling training facility, as well as killing “up to 180 foreign mercenaries.”

Reuters could not independently verify the casualties reported by either side.

The attack on the Yavoriv International Centre for Peacekeeping and Security, a base just 15 miles (25 km) from the Polish border that has previously hosted NATO military instructors, brought the conflict to the doorstep of the Western defence alliance.

Russia had warned on Saturday that convoys of Western arms shipments to Ukraine could be considered legitimate targets.

Britain called the attack as a “significant escalation,” and U.S. Secretary of State Antony Blinken responded with a post on Twitter saying “the brutality must stop.”

White House national security adviser Jake Sullivan, speaking on CBS’s “Face the Nation”, warned any attack on NATO territory would trigger a full response by the alliance.

Regional governor Maksym Kozytskyy said Russian planes fired around 30 rockets at the Yavoriv facility.

Russian defence ministry spokesperson Igor Konashenkov said Russia had used high-precision, long-range weapons to strike Yavoriv and a separate facility in the village of Starichi.

“As a result of the strike, up to 180 foreign mercenaries and a large amount of foreign weapons were destroyed,” he said.

The 360-square km (140-square mile) facility is one of Ukraine’s biggest and is the largest in the western part of the country, which has so far been spared the worst of the fighting.

Ukraine, whose aspirations to join NATO are a major irritant to Russian President Vladimir Putin, held most of its drills with Western countries at the base before the invasion. The last major exercises were in September.

In the weeks before Russia’s Feb. 24 invasion, the Ukrainian military trained there, but according to Ukrainian media all foreign instructors left in mid-February, leaving behind equipment.

“The dining room and dormitory were destroyed. So were the barracks,” said Colonel Leonid Benzalo, an officer in the Ukrainian medical reserve who was thrown across the room by one of the blasts. “The most important thing is we’re still alive,” he told Reuters after treating the wounded there.

While Western nations have sought to isolate Putin by imposing harsh economic sanctions and have been supplying Ukraine with weapons, the United States and its allies are concerned to avoid NATO being drawn into the conflict.

“There are no NATO personnel in Ukraine,” the NATO official said, when asked if anyone from the alliance was at the base.