Russian and Ukrainian delegations were to meet in Paris for talks on Wednesday in a bid to calm tensions over Moscow’s military buildup on its neighbour’s frontier.
The high-level meeting, also attended by senior diplomats from France and Germany, brings together the four parties to the 2015 Minsk peace accords which sought to end fighting in eastern Ukraine.
Russia is represented by vice-prime minister Dmitri Kozak and Ukraine by presidential advisor Andriy Yermak, with diplomatic advisors to President Emmanuel Macron and German Chancellor Olaf Scholz also taking part in the so-called "Normandy Format" talks from 11 GMT.
Each side is expected to brief the media later in the day, with an aide to Macron saying earlier in the week that France and Germany would attempt to move Russia and Ukraine along "a path to de-escalation."
The meeting comes after Western powers on Tuesday kept up their warnings of massive economic sanctions in case of any Russian attack on pro-Western Ukraine.
Moscow has more than 100,000 troops massed on its neighbour's border and is carrying out or preparing military drills with thousands more in Belarus and the Crimean peninsula, which it annexed from Ukraine in 2014.
"If there is an attack, there will be retaliation and the price (for Russia) will be very high," Macron said alongside Scholz on a visit to Berlin.
The French leader is set to talk by phone with Putin on Friday.
Play the game
British Prime Minister Boris Johnson echoed the threat, saying sanctions would be "heavier than anything we've ever done."
American President Joe Biden also warned that there could be personal sanctions against Russian leader Vladimir Putin if fighting began, while US officials trailed cutting Moscow off from US high-tech exports.
The US has also been drawing up plans to shore up European gas supply should Russia cut off shipments through its pipelines in response to any sanctions.
The White House announced Tuesday Biden would meet with the emir of gas-rich Qatar, Sheikh Tamim bin Hamad Al Thani, on January 31 to discuss, among other issues, "ensuring the stability of global energy supplies."
The French "de-escalation" plan, as detailed by an aide to Macron on Monday, would entail both sides taking steps to build confidence
These would include Ukraine's parliament delaying a law governing the status of Russian-backed separatist provinces in the east of the country, which Moscow sees as violating previous commitments.
Russia in turn would back "humanitarian measures" such as prisoner exchanges in eastern Ukraine and the opening of check-points manned by the separatists.
France also would push for "a public statement from the Russians about their intentions that reassures everyone," the aide said.
"We will check to see if the Russians are prepare to play the game. We have indications that they are, but we can't be certain," the advisor said on condition of anonymity.
The leaders of Russia, Ukraine, France and Germany last met in Paris in December 2019 while senior diplomatic advisors from the four countries met in Berlin in January last year with no breakthrough.