Paris - French President Francois Hollande on Thursday described the possible presence of Russian troops in eastern Ukraine as "intolerable and unacceptable" and called on Moscow to "respect the sovereignty of Ukraine" and "stop supporting the separatists".
"If it is confirmed that Russian troops are present on Ukrainian soil, this would be intolerable and unacceptable", he said in a speech to France's annual conference of ambassadors, which brings together all of the country's heads of embassies and diplomatic missions.
The president warned that European Union sanctions against Russia "will be maintained, or reinforced, if the escalation continues".
The US government has accused Moscow of sending soldiers into Ukraine without telling them or their families where they are going.
Calling the Ukrainian conflict "one of the worst crises since the end of the cold war", Hollande also urged the Ukrainian government to show "restraint" in fighting separatist militias that have seized parts of the country's eastern regions.
"The Ukrainian authorities must show restraint in their military operations and agree a significant decentralization to the benefit of the Russian-speaking regions", he said.
Ukrainian President Petro Poroshenko on Wednesday proposed a road map to solve the bloody conflict.
This would involve re-establishing government control over the border with Russia, a bilateral ceasefire and the release of prisoners. Hollande called on Russia to pressure the separatists to accept such a ceasefire.
Poroshenko and Russian President Vladmir Putin recently agreed to revive a so-called contact group made up of the Russia, Ukraine and the organisation for security and co-operation in Europe (OSCE).
Hollande said he was ready to organise with Germany another meeting of the Russian and Ukrainian presidents. The two heads of state have met twice this year, first on 6 June at France's D-Day commemorations in Normandy, and then at a summit in the Belarusian capital of Minsk on Tuesday.
German Chancellor Angela Merkel on Wednesday held telephone calls with both Putin and Poroshenko as part of her ongoing efforts to mediate a diplomatic solution.
The French president on Thursday said Russia's behaviour "challenges the principles on which our collective security has been founded since the end of the cold war" and was incompatible with being a 21st century power.
"Russia cannot, at the same time, aspire to be a recognized power of the 21st century and not respect its rules", he said. "It's obviously above all up to the Russian president to resolve this contradiction."