Poroshenko should have been among nine heads of state at the inauguration ceremony in Zagreb on Sunday.
"President Poroshenko is the supreme commander of the army and must be in the country," where a ceasefire agreement between government forces and pro-Russian separatists takes effect at midnight, a source at the embassy told Hina.
The situation became uncertain after a column of tanks, armoured personnel carriers and troops entered Ukraine on Friday, heading for the cities of Debaltseve and Luhansk.
"If the ceasefire fails, President Poroshenko has announced that he will declare a state of emergency in the country," the source said.
The Ukrainian army said on Saturday that there was no letup in the offensive by pro-Russian rebels in eastern Ukraine, who are trying to take as much territory as possible before the ceasefire takes effect at midnight tonight, or 2300 hours Central European Time.
The ceasefire agreement foresees the creation of a buffer zone and the withdrawal of heavy weapons, responsible for the deaths of more than 5,000 people in the conflict that broke out nearly a year ago.
According to the authorities in Kiev, seven Ukrainian soldiers have been killed and 23 wounded in the last 24 hours.