The United States have warned Russia that NATO will respond with "full force" if a country in the organisation is attacked.
The comments come after a major missile attack on a military facility in western Ukraine just 10km away from the Polish border.
Poland is a member of NATO.
At least 35 people are known to have been killed and more than 130 suffered injuries so far, according to officials in Yavoriv.
They said that up to 30 missiles were fired at the International Center for Peacekeeping and Security on Sunday morning.
It led to US national security adviser Jake Sullivan warning that if Russia "takes a shot on" NATO territory, the bloc would respond with its "full force".
"If there is a military attack on NATO territory it would cause the invocation of Article 5, and we would bring the full force of the NATO alliance to bear in responding to it," he told CBS.
"All I will say is that if Russia attacks, fires upon, takes a shot at NATO territory, the NATO alliance would respond to that".
Mr Sullivan's comments come hours after an American photojournalist and film-maker was shot dead in Ukraine by Russian forces.
Brent Renaud was killed in Irpin, near Kyiv, on Sunday by Russian forces, Kyiv's police chief Andriy Nebytov said.
Renaud had a glittering award tally for his previous work, including for the likes of the New York Times.
A statement from the paper said: "We are deeply saddened to hear of Brent Renaud's death. Brent was a talented photographer and film-maker who had contributed to the New York Times over the years.
"Though he had contributed to the Times in the past (most recently in 2015), he was not on assignment for any desk at the Times in Ukraine.
