"We condemn the Russian Federation's missile attack on the International Center for Peacekeeping and Security in Yavoriv, close to Ukraine's border with Poland," Blinken said on Twitter on Sunday.
"The brutality must stop," he added.
Thirty-five people were killed and 134 others wounded when Russian missiles hit the large Ukrainian base near the Polish border on Sunday, the Reuters news agency reported, citing a local official.
If Russia attacks NATO territory, allies will respond: US national security adviser
White House national security adviser Jake Sullivan said in a media interview on Sunday that any military attack on NATO territory would lead to the invocation of Article 5 of the Washington Treaty, which guarantees collective defence by stating that an armed attack against one NATO member state “shall be considered an attack against them all."
Sullivan said: "If Russia attacks, fires upon, takes a shot at NATO territory, the NATO alliance would respond to that."
He told broadcaster CBS that President Joe Biden "has been clear repeatedly" that the United States would work with allies "to defend every inch of NATO territory, and that means every inch."
US National Security Advisor Jake Sullivan. Photo: EPA/MICHAEL REYNOLDS
Polish President Andrzej Duda told reporters after a phone call with Biden earlier this month that Poland, a NATO member and a staunch European ally of the United States, was safe amid Russia's brutal war against Ukraine.
Source: PAP, Reuters, cbsnews.com