US President Donald Trump told reporters that US teams were headed to Moscow to discuss the 30-day ceasefire plan with Ukraine.
Trump was meeting with the Irish prime minister in the Oval Office, when he was asked questions about whether Russia would accept the ceasefire proposal.
Kyiv accepted the plan on Tuesday.
"It's up to Russia now," Trump told reporters, adding that he could impose "devastating" sanctions on Russia that could punish the country.
"I hope that's not going to be necessary," he said, adding: "I can do things financially that would be very bad for Russia. I don't want to do that because I want to get peace."
US negotiators en route to Russia to discuss ceasefire plan, Trump says
US President Donald Trump said US officials were headed to Russia "right now" to talk about a ceasefire plan to end the war in Ukraine.
Ukrainian and US officials agreed on the ceasefire on Tuesday, along with a restoration of US military aid and intelligence to Kyiv, after more than eight hours of talks in the Saudi Arabian city of Jeddah.
"People are going to Russia right now as we speak. And hopefully we can get a ceasefire from Russia," Trump told reporters in the Oval Office during a meeting with Irish Prime Minister Micheal Martin.
"I've gotten some positive messages, but a positive message means nothing," Trump said in response to questions from reporters about progress on Ukraine. "This is a very serious situation."
The Kremlin said Wednesday it was still awaiting details from Washington about the ceasefire proposal, with Reuters news agency quoting senior Moscow officials as saying the deal would have to take into account Russia's advances and address its concerns.