The Russian prime minister, Dmitry Medvedev, has said Moscow would respond to any new western sanctions "asymmetrically," possibly preventing airlines flying through Russian airspace, if the US and EU press on with "the temptation to use force in international relations".
In an interview with Russian daily Vedomosti published on Monday, Medvedev said: "If there are sanctions related to the energy sector, or further restrictions on Russia's financial sector, we will have to respond asymmetrically."
The EU has agreed on new economic sanctions on Russia, which should come into force on Monday, but said they could be suspended to allow Moscow to show it was resolving the conflict in Ukraine that has killed nearly 3,000 people and caused the worst confrontation between Russia and the west since the cold war.
A ceasefire was agreed by envoys from Ukraine, the separatist leadership, Russia and Europe's OSCE security watchdog on Friday but has been tested by fighting in eastern Ukraine.
Shelling resumed near the port of Mariupol on the Sea of Azov on Saturday, just hours after Vladimir Putin and his Ukrainian counterpart, Petro Poroshenko, had agreed in a phone call that the truce was holding. One woman was killed and at least four others wounded in fighting overnight into Sunday.
Medvedev said the crisis was continuing and that all sides should seize the proposals for a peace plan put forward by Putin, which would leave pro-Russia rebels in control of large areas of eastern Ukraine.
"Unfortunately, the Ukraine crisis continues unabated, which is a terrible tragedy," Medvedev said.
"Hopefully it will be resolved, provided the Ukrainian leaders show goodwill and seize the proposals put forward by Russia."