More than 8,000 people have died in Ukraine's fight against separatist rebels in the east since April 2014.
Russia backed the rebels, fearing that the pro-Western course of the new Kiev government would make it a NATO ally.
"The decision on the non-alliance policy which was announced by the previous government is criminal in terms of security and strategic interests of our state," Poroshenko said, citing an opinion poll showing 60 per cent of Ukrainians supporting potential NATO membership.
Before the February 2014 uprising which toppled pro-Russian President Viktor Yanukovych, the majority of Ukrainians opposed joining NATO, but many now see the alliance as their best defense from a foreign military threat.
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NATO officials say Ukraine needs to enact political, economic, social and military reforms to be considered for membership.
Stoltenberg told the meeting today that "Ukraine can rely on NATO" and that "NATO provides Ukraine with political and practical support." He lauded the Kiev government for sticking to the Minsk cease-fire agreements and called on Moscow and the rebels to respect the deal.
NATO has expressed firm support for Ukraine in its conflict with Russia, but alliance member states, including the US, have declined its request for offensive weapons.