Ukrainian President Petro Poroshenko said here Saturday that the crisis with Moscow over its alleged support for mainly ethnic-Russian rebels fighting Kiev's forces in eastern Ukraine is approaching "full-sale war."

"I think that we are very close to the point of no return," he said at a press conference at a European Union summit in Brussels, where Polish Prime Minister Donald Tusk and Italian Foreign Minister Federica Mogherini were chosen Saturday as president of the European Council and EU foreign policy chief, respectively.

"Point of no return is full-scale war," Poroshenko added ominously.

He reiterated Kiev's and NATO's allegations that Russian troops and equipment have illegally crossed the border into eastern and southeastern Ukraine and warned that a point of no return would be reached if they take any offensive action.

NATO has released what it says are satellite images supporting the accusations.

The Ukrainian president, however, also said that great efforts are being made to ensure such action does not occur and to solve the crisis diplomatically.

In that regard, he expressed confidence that a cease-fire agreement can be reached in trilateral negotiations next week involving the Organization for Security and Co-Operation in Europe, Russia and Ukraine.

In a tense emergency meeting of the U.N. Security Council on Thursday, Kiev's representative, Oleksandr Pavlichenko, accused Russia of launching a "direct military invasion" of Ukraine.

The U.S. and Russian ambassadors also engaged in a war of words at the gathering.

"Russia has to stop lying and has to stop fueling this conflict," U.S. Ambassador Samantha Power said during Thursday's session.

"The mask is coming off. We see Russia's actions for what they are: a deliberate effort to support and now fight alongside illegal separatists in another sovereign country," she said, referring to the satellite imagery taken on Tuesday.

Russia's U.N. representative, Vitaly Churkin, criticized both Kiev and Washington.

"The Kiev authorities have torpedoed all political agreements on resolving the crisis. The only thing we're seeing is a fight against dissent," the Russian diplomat said.

Kiev refers to the eastern militias as terrorists and Western governments usually call them separatists, though their original demands were for a federal system that would give Ukraine's regions greater autonomy.

Churkin denied that Russian forces are backing the rebels in eastern Ukraine, although he acknowledged the presence of Russian volunteers in that region.

"There are Russian volunteers in eastern parts of Ukraine. No one is hiding that," he said, before asking Power to explain the presence of what he said were dozens of U.S. intelligence personnel in Kiev.

The veteran diplomat said he wanted to "send a message to Washington: Stop interfering in the internal affairs of sovereign states." EFE