He said this while answering questions of the PACE deputies, Ukrainska Pravda reports.
Poroshenko said that on Wednesday the trilateral contact group, composed of representatives of Ukraine, Russia, and the OSCE, had a meeting.
According to Poroshenko, on Friday, the group will meet again.
There were no representatives of the terrorists at the last meeting, "but it is not necessary [for them be at the meeting - Ed.]."
"Half an hour ago, they expressed an intention to participate in the meeting on Friday. This is a very important day, because if our conditions for the peace plan are not accepted, we will make a very important decision."
Once again he urged Europe to be united regarding Ukrainian problems and noted that "as for now, there is no enough support."
"It can happen to every country, if Europe does not unite around this issue," the President said.
Poroshenko reminded that Ukraine suffers losses after the announcement of the ceasefire.
Also, "under separatists' fire" Ukrainian forces are trying to restore water and electricity in Sloviansk, Kramatorsk and other cities, where people are deprived of them for more than 10 days.
"Ukraine demonstrates its desire for peace not only by words but by actions. And we have already paid a very high price for this desire," Poroshenko said.
As it was reported earlier, Poroshenko calls "insufficient" Russia's support of the peace plan.