Russia is "very likely" to invade Ukraine and might only be deterred by "enormous sanctions", the chair of the US House intelligence committee said on Sunday.
Adam Schiff also said an invasion could backfire on Moscow, by drawing more countries into the Nato military alliance.
"I also think that a powerful deterrent is the understanding that if they do invade, it is going to bring Nato closer to Russia, not push it farther away," he said.
This week, Joe Biden told Vladimir Putin the US would impose serious sanctions if Russia attacked.
Talks are scheduled. But amid tensions heightened by both sides' possession of nuclear weapons, Russia's foreign minister, Sergei Lavrov, said that if "the west continues its aggressive line, Russia will be forced to take all necessary measures to ensure strategic balance and eliminate unacceptable threats to our security".
Russia has for years complained about Nato encroachment. Ukraine is not a member of the alliance, which guarantees collective defence, but Nato has expanded eastwards since the fall of the Soviet Union and Kiev is urgently seeking admission.
Russia invaded Ukrainian territory in 2014, annexing Crimea.
The US has supplied "small" arms to Ukraine.
On CBS's Face the Nation, Schiff was asked what would stop Putin ordering an invasion by Russian troops gathered near the border.
"I think that it would require enormous sanctions on Russia to deter what appears to be a very likely Russian invasion of Ukraine again," Schiff said. "And I think our allies need to be solidly on board with it. Russia needs to understand we are united in this."
An invasion, Schiff said, would see "more Nato assets closer to Russia. [It] will have the opposite impact of what Putin is trying to achieve".
Schiff said he had "no problem" with "going after Putin personally", but thought "sector-sized sanctions will be the most important".
Asked if he thought scheduled talks had any chance of averting an invasion, he said: "I fear that Putin is very likely to invade. I still frankly don't understand the full motivation for why now he's doing this, but he certainly appears intent on it unless we can persuade him otherwise.
"And I think nothing other than a level of sanctions that Russia has never seen will deter him, and that's exactly what we need to do with our allies."