He added that this made Europe’s solidarity with Ukraine “crucially important for all of us.”

Russia has dismayed European leaders by repeatedly denying that it has sent troops or military hardware into Ukraine. After the Ukrainian authorities released videos on Tuesday of captured Russian troops, Moscow conceded that some of its soldiers had crossed into Ukraine but said they had done so “by accident.”

Rebel leaders say Russian servicemen are fighting in Ukraine during their holiday leave. Aleksandr Zakharchenko, a separatist leader in Donetsk, said earlier this week that these soldiers “would rather take their vacation not on a beach but with us, among brothers, who are fighting for their freedom.”

Russia’s evasions and denials in response to mounting evidence of its direct involvement in supporting pro-Russian separatists has left even Europe’s more cautious leaders, notably Ms. Merkel, ready to endorse further sanctions. Ms. Merkel, the dominant figure in European policy-making, said early Sunday that Germany still favored a negotiated settlement and that Europe needed to keep the pressure on Russia with additional sanctions. “We need to do something to clearly demonstrate what are the values we defend,” she said.

She said that Russia’s opaque political system made it difficult to assess whether sanctions already in place were affecting Russian decision-making but added: “I would say they are.”

Ms. Merkel has spoken regularly with Mr. Putin, by telephone during the crisis but has had no success in curbing Russia’s support for the rebels, who had been losing ground in the face of a Ukrainian offensive. Now, reinvigorated by new arms and fighters from Russia, the rebels are expanding territory under their control.

Mr. Barroso said that he, too, had spoken by phone with Mr. Putin and “urged him to change course” during a “long and frank” conversation on Friday.

While not directly accusing Russia of sending soldiers into Ukraine, as Mr. Poroshenko and NATO have done, Mr. Barroso said Russian moves to feed fighting in eastern Ukraine were “simply not the way responsible, proud nations should behave in the 21st century.” Further sanctions, Mr. Barroso said, would “show to Russia’s leadership that the current situation is not acceptable and we urge them to come to reason.”