Turkey’s shooting down of a Russian jet fighter is a “critical moment” for the U.S. and its allies to stand up to Moscow, Sen. Marco Rubio (R-Fla.) said Tuesday.

Hours after the Kremlin condemned Turkey’s decision to shoot down the warplane for entering its airspace, Rubio warned against letting Russia get the upper hand. 

“It's important for us to be very clear that we will respond and defend Turkey if they come under assault from the Russians. Otherwise the entire NATO alliance comes into question,” Rubio, who is running for president, said on Fox News.

“If Russia believes that they can respond and retaliate against Turkey because NATO is not going to do anything about it, what they are basically doing is making the argument that NATO is no longer viable, that it is a feckless alliance.”

The incident was the first time a NATO member has shot down a Russian target since the Cold War, and it threatened to escalate the political tensions surrounding the 4-year-old civil war in Syria.

Russian President Vladimir Putin was quick to condemn Turkey’s action, calling it a “stab in the back” from “accomplices of terrorists.”

Forceful U.S. action would keep a lid on possible retaliation, Rubio said.

“Vladimir Putin is a gangster and a criminal, but he is also a geopolitical actor who makes decisions on a cost-benefit analysis,” he said on Fox. “He will have to do something to save face, but ultimately he is not going to test the alliance if he believes the alliance is going to stand up to him, because he will lose in that confrontation, and that would be an even bigger setback for him.”

President Obama stood by Turkey on Monday, insisting that the NATO ally “has the right to defend its territory and its airspace.”

“I think it’s very important right now for us to make sure that both the Russians and the Turks are talking to each other to find out exactly what happened and take measures to discourage any kind of escalation,” Obama added, during a press conference at the White House.