Prior to Russia’s full-scale invasion of Ukraine, the facility hosted NATO drills and U.S. troops used it to train Ukrainian forces on the deployment of anti-tank missiles and other weaponry. The base has been described as “a vital link in the pipeline to get weapons from NATO allies into Ukraine.”

It’s unclear whether there were any foreign instructors at the complex at the time of the strike early Sunday morning, Reuters reported. Just two weeks before Russia launched its invasion, the Pentagon withdrew around 160 U.S. military trainers from Ukraine.

“Russia has attacked the International Center for Peacekeeping and Security near Lviv,” Oleksii Reznikov, Ukraine’s defense minister, wrote on Twitter. “Foreign instructors work here. Information about the victims is being clarified. This is a new terrorist attack on peace and security near the E.U.-NATO border.”

Russia’s strike on the Ukrainian base—believed to be the closest Russian missiles have come to the border of a NATO member thus far—hit a day after Moscow said it views Western arms shipments flowing into Ukraine as “legitimate targets.”

NATO Secretary-General Jens Stoltenberg said in a recent interview that he would see any Russian attack on supply lines of countries providing weapons to Ukraine—which is not a NATO member—as a dangerous escalation.

“Russia is the aggressor and Ukraine is defending itself. If there is any attack against any NATO country, NATO territory, that will trigger Article 5,” Stoltenberg added, referring to the NATO treaty’s self-defense clause.

Shortly following Moscow’s warning on Saturday, U.S. President Joe Biden authorized another $200 million in arms and equipment for Ukraine, a package that reportedly includes Javelin anti-tank missiles and Stinger anti-aircraft missiles.

Over the past year, according to the New York Times, the Biden administration has approved $1.2 billion in weapons for Ukraine.

“The weapons come from existing U.S. military stockpiles in Europe and are flown to neighboring countries such as Poland and Romania, where they are shipped overland into western Ukraine,” the Times noted Saturday. “In less than a week at the beginning of the Russian assault, the United States and NATO pushed more than 17,000 anti-tank weapons, including Javelins, into the hands of Ukrainian commanders.”