Russia says it invaded Ukraine to halt Kyiv's ambitions to join NATO, the U.S.-led defensive alliance, which it saw as a major security threat. It also accuses Kyiv of persecuting ethnic Russians and their culture in Ukraine.

Despite his optimism, Trump did not shy away from estimating that there was a "25% chance the meeting is a failure."

Oil prices were stable on Thursday as investors weighed the potential impact of Friday's U.S.-Russia summit on Ukraine on Russian crude flows, after U.S. President Donald Trump warned of "severe consequences" for Russia if it does not agree to peace.