WASHINGTON -- US President Donald Trump on Tuesday said he is considering "very serious" economic sanctions against Russia if it rejects a ceasefire in the conflict in Ukraine that has lasted three and a half years.

"It's very, very serious what I have in mind, if I have to do it, but I want to see it end," Trump was quoted as saying by media reports, in response to a reporter asking if Russian President Vladimir Putin would face consequences should he fail to agree to a ceasefire.

"We have economic sanctions. I'm talking about economic because we're not going to get into a world war," he added.

Trump has long threatened sanctions against Russia and is trying to mediate a one-to-one meeting between Putin and Ukrainian President Volodymyr Zelensky, for which the Kremlin has said conditions have not been ready.

Russian Foreign Minister Sergei Lavrov said Thursday that deploying foreign troops in Ukraine is unacceptable to Russia, as the United States and Europe work to provide security guarantees for Kiev.

The foreign minister also noted on Aug. 19 that Russia does not reject any formats, either bilateral or trilateral, on resolving the conflict in Ukraine.