"I think that we are very close to the point of no return," Poroshenko said at a press conference at a European Union summit in Brussels.
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 released satellite images supporting the accusations.
The Ukrainian president, however, said that great efforts were being made to ensure such action does not occur and to solve the crisis diplomatically.
He, however, expressed confidence that a ceasefire agreement can be reached in trilateral negotiations next week, involving the Organisation for Security and Co-Operation in Europe(OSCE), Russia and Ukraine.
In an emergency meeting of the UN Security Council Thursday, Kiev's representative, Oleksandr Pavlichenko, accused Russia of launching a "direct military invasion" in Ukraine.
The US and Russian ambassadors also engaged in a war of words at the gathering.
"Russia has to stop lying and fueling the conflict," US Ambassador to the UN Samantha Power said during Thursday's session.
"The mask is coming off. We see Russia's effort to support and now fight alongside illegal separatists in another sovereign country," Power added, referring to the satellite imagery taken Tuesday.
Russia's UN representative, Vitaly Churkin, criticised 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.
He also asked Power to explain the presence of dozens of US intelligence personnel in Kiev.