Doug Stanglin

USA TODAY

Russian Prime Minister Dmitry Medvedev warned Monday that Russia may close its airspace to flyovers by Western airlines if the European Union imposes sanctions this week.

EU ministers agreed on a new round of sanctions over the Ukraine conflict during a side meeting at a NATO summit last week in Wales.

The measures were adopted Monday by the European Council, but enforcement was delayed to assess a cease-fire signed Friday.

The sanctions will be implemented "in the next few days," European Council President Herman Van Rompuy said Monday.

The latest sanctions include a ban on loans and financing from the EU to major Russian energy firms — including Rosneft, Transneft and Gazprom Neft, Radio Free Europe/Radio Liberty reported.

"If there are sanctions related to the energy sector or further restrictions on Russia's financial sector, we will have to respond asymmetrically," Medvedev said in an interview published Monday in the Russian business newspaper Vedomosti.

"We could impose transport restrictions," Medvedev said. "We believe we have friendly relations with our partners, and foreign airlines of friendly countries are permitted to fly over Russia. However, we'll have to respond to any restrictions imposed on us."

Shutting down air corridors would be an expensive blow to Western airlines that regularly traverse the world's largest country on flights between Europe and Asia.

The threat of a Russian flyover ban surfaced in July after Western sanctions hit Aeroflot's low-cost subsidiary Dobrolet.

The shaky Ukrainian cease-fire appeared to hold, despite reports of shelling over the weekend around the southeastern port city of Mariupol and near Donetsk in eastern Ukraine.

The Organization of Security and Cooperation in Europe said the shelling violated the cease-fire, but it did not seem enough to trigger a collapse of the agreement.

Ukrainian President Petro Poroshenko visited Mariupol on Monday and vowed to defend it from Russian-backed rebels. He said Kiev had ordered the city, on the Sea of Azov, to be reinforced with tanks, rocket launchers and air defenses.

Also Monday, the Kremlin said Poroshenko and Russian President Vladimir Putin spoke by phone and "continued to discuss steps helping peaceful settlement in southeastern Ukraine."

"The dialogue will continue," the Kremlin said without elaborating.

Separatists, reinforced by what NATO says is Russian military equipment and soldiers, had been advancing on Mariupol when the cease-fire was signed. Russia has steadfastly denied any military involvement in Ukraine.

Nearly 3,000 people have died in fighting between Ukrainian armed forces and separatist groups in eastern Ukraine since April. Poroshenko said the rebels have released 1,200 Ukrainian soldiers under the terms of the cease-fire.