Sen. Jon Tester on Thursday labeled President Vladimir Putin a war criminal and said the U.S. would do everything in its power to ensure Ukraine has the resources it needs to defeat Russian aggression.

Tester didn't identify a “red line” of Russian actions that would potentially prompt a military response from the U.S., but he did say the U.S. and NATO would continue to explore all options and have already provided Ukraine with billions of dollars in military aid.

“If you look at the pictures on the news at night and you see what Putin is doing to that country, this guy is a war criminal and he's going to be held accountable for his actions, and I think he knows it,” Tester told the Missoula Current. “It's like having a wild animal cornered. You don't know where he's going to go.”

Tester will join the chair of the Senate Armed Services Committee in a trip to Poland in the coming days to meet with representatives from Poland, the Baltics and Urkaine on what they need to move forward.

Sen. Steve Daines recently traveled to Poland and Germany as well.

“What we've seen with Russia, they're not as good as they thought they were and Ukraine could win this war,” Tester said. “At this moment in time, we're doing everything possible and if there's something we're missing, hopefully we'll find out about it next week and we'll do that.”

Tester said the U.S. would continue to supply Ukraine with “everything it needs” to fend off the Russian invasion. However, he said a no-fly zone would likely be ineffective given Russia's current military tactics.

The U.S. looks to avoid direct military engagement with Russian forces, Tester added.

“The deal with the red line, whatever it may be, is that you're dealing with Russia and they're a nuclear power, and where does this stop,” Tester said. “A nuclear conflict brings in the whole world. Right now, the U.S. is helping both from an equipment standpoint and an information standpoint.”

Tester added that the sanctions placed on Russia are working and in time, they could alter the outcome of the war.

“These sanctions we put on Russia are having some serious effects,” he said. “If we can get the generals to be able to influence him, and the oligarchs, which I think could happen – I'm not saying it will but it could – we'll likely see a different outcome to this war.”