Any country offering its airfields to Ukraine for attacks on Russia may be considered as having entered the conflict, a Russian defense ministry spokesman was quoted as saying on Sunday.
"The use of the airfield networks of these countries to base Ukrainian military aircraft and their subsequent use against the Russian armed forces may be regarded as the involvement of these states in an armed conflict," Interfax news agency quoted spokesman Igor Konashenkov as saying.
Moscow has been increasingly upping the rhetoric as fighting in the Eastern European country protracts and Russian forces continue facing fierce Ukrainian resistance.
On Saturday, Russian President Vladimir Putin equated Western sanctions to an act of war.
Last Monday, Russia's foreign ministry warned those supplying lethal weapons to Ukraine will bear responsibility should they be used during Russia's military campaign there.
The ministry added that the steps the European Union has taken against Russia will not be left without a harsh response.
Earlier on Sunday, French President Emmanuel Macron urged Putin to end Russia's military operations in Ukraine and to ensure the protection and security of Ukraine's nuclear sites, according to an official from Macron's office.
The official confirmed that Macron had held a phone exchange of nearly two hours on Sunday with Putin.
Russian media had earlier reported the latest talks between Putin and Macron. Macron has stayed in regular contact with Putin but, as with other international efforts, has yet to persuade Moscow to call off the campaign as it entered its 11th day.
Putin said on Sunday that his campaign in Ukraine was going according to plan and would not end until Kyiv stopped fighting, as efforts to evacuate the heavily bombarded city of Mariupol failed for a second day in a row.
Putin made those comments in a phone call with Turkish President Tayyip Erdogan, who appealed for a ceasefire in the conflict that the United Nations says has created the fastest-growing refugee crisis in Europe since World War Two.
Meanwhile, Ukrainians fleeing the town of Irpin just outside Kyiv were caught in shelling by Russian forces on Sunday and forced to dive for cover, Reuters witnesses said.
Irpin, some 25 km (16 miles) northwest of the capital, has seen intense fighting in recent days. Russia's military is closing in on Kyiv, home to around 3.4 million people before the invasion sparked a mass exodus of civilians.
Irpin residents scurried along pavements clutching children, luggage and pets as they made their way to waiting buses and cars that would take them further from the clashes.
Soldiers and fellow residents helped elderly men and women who were falling behind. Some people crouched down when explosions went off nearby, apparently from mortar rounds.