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He also acknowledged Kiev lacked the resources to regain control by military means.
Poroshenko said he had spoken with U.S. President Barack Obama about the possibility of his joining talks on the territories where pro-Russian separatists launched a rebellion in April.
"The most important thing is to turn a fragile ceasefire into a stable peace and return previously occupied territories under the control of Ukrainian authorities," Poroshenko said.
He told a news conference he would meet French President Francois Hollande, German Chancellor Angela Merkel and Russia's Vladimir Putin on January 15. in Kazakhstan's capital, Astana.
He said Kiev would not compromise on sovereignty and wanted east Ukraine, where over 4,700 people have been killed since April, as well as Crimea, annexed by Russia in March, back under its control. But a military solution was not possible.
"We haven't got the resources for an offensive today."
Last week, Ukrainian authorities and separatists exchanged hundreds of prisoners of war, in a step towards a de-escalation of the conflict and which coincided with Russia being hit by deep economic troubles linked to lower oil prices and Western economic sanctions.
"Ukraine is paying an enormous price because of aggression, but Russia is also paying heavily," Poroshenko said.
Poroshenko said Russia had agreed to supply Ukraine with power and coal, in another sign of an easing in tensions.
The uprising by separatists in Ukraine's east began a month after Russia annexed the Black Sea peninsula of Crimea from Ukraine in March, following the popular overthrow of Ukraine's Moscow-backed president.
The pro-Western government in Kiev accuses Russia of orchestrating the rebellion in Ukraine's east, a charge denied by Moscow.
Talks between Russia, Ukraine and the Organization for Security and Cooperation in Europe brokered a ceasefire between Kiev and the rebels in September. The truce was repeatedly broken by both sides but violence has eased significantly in December.
On Monday, Poroshenko signed into law Kiev's renunciation of its neutral status in pursuit of NATO membership.
Russia said last week NATO was turning Ukraine into a "frontline of confrontation" and threatened to sever remaining ties if Ukraine's hopes of joining NATO were realised.
"When Ukraine will meet those (NATO) criteria - possibly within the next 5-6 years - then the people of Ukraine will make their choice," Poroshenko said.