"The territorial integrity and sovereignty of Ukraine are non-negotiable," President Francois Hollande said after a meeting with Ukraine's former foreign minister Petro Poroshenko and Vitali Klitschko, the ex-boxer and leading figure of the protest movement that ousted the country's pro-Moscow president.
Russia has said it would respect a decision by lawmakers in Ukraine's flashpoint Crimea region to renounce ties with Kiev and stage a referendum on switching to Kremlin rule.
Hollande said the West's response to the crisis would be "modulated according to the situation."
"Our aim is to also always leave open the door for negotiations so that Russia can enter into talks if it decides to do so," he said.
Hollande said the "international community, Europe and France must work to preserve the territorial integrity of Ukraine."
He also warned that it was necessary to prevent a dangerous precedent.
"There are a lot of countries which could get worried if a precedent were set for breaching borders and territorial integrity," Hollande said.
Klitschko shared that view saying that "the instability threatens not only Ukraine but also the whole region."
To support the Ukrainians, Poroshenko called for "using all the sanctions possible" against Russia, saying the solution to the crisis "is not to be found in Kiev, or in Crimea but in Moscow."
He also said before any political solution could be reached there must be "the complete withdrawal of the foreign army" in Crimea, where Russian forces have taken effective control over the past week.