French Foreign Minister Laurent Fabius said financial sanctions and travel restrictions could go into effect within days unless Moscow accepted a U.S. proposal for discussions to end the crisis in Ukraine, the Washington Post reported.
Western officials were to meet in London Tuesday to forge details of the sanctions plan.
U.S. Secretary of State John Kerry has told Russian officials they must halt their advance in the autonomous, pro-Moscow Crimea and open dialogue with Ukraine's new government in Kiev before he would visit Moscow for talks.
On Monday, Russian Foreign Minister Sergei Lavrov called the U.S. proposal "unsatisfactory."
Fabius told a French radio station that, absent a positive Russian response, "there are a series of sanctions that can be taken as early as this week."
Meanwhile, pro-Russian forces tightened their grip in the Crimean peninsula, where witnesses in the regional capital of Simferopol reported troops seized the prosecutor's office and the railway administration, the Post said.
Elsewhere in Crimea, a convoy of about 40 Russian military trucks was seen Tuesday near Saky, said Vladislav Seleznev, a Ukrainian Defense Ministry spokesman.
At Simferopol's civilian airport, flights to Istanbul, Turkey, and Kiev were canceled Tuesday. Only flights to Moscow were available.
In Kiev, Interim President Oleksander Turchinov said the country would have start from scratch to rebuild its military and that veterans would be tapped to form a new national guard, the Post said. He said Ukraine only had 6,000 combat-ready infantry, compared to 200,000 Russian troops on Ukraine's eastern border.
Turchinov blamed Ukraine's ousted President Viktor Yanukovych for the military's deterioration.