The brief statement gave no details and there was no immediate reaction from the Russia-backed separatists whom Ukrainian forces have been fighting since April.
Rebels have been battling Kiev's forces in the mainly Russian-speaking Donbas region, which is home to most of Ukraine's heavy industry and accounts for about 18 percent of the country's economic output.
On Monday, Ukraine's military pulled its forces back from defending a vital airport in the east against what is described as a column of Russian tanks, with Poroshenko accusing Moscow of "direct and open aggression".
The withdrawal from the civilian airport outside the city of Luhansk was the latest in a string of reverses for Ukrainian forces fighting pro-Russian rebels who Kiev says have the direct support of hundreds of Russian troops and armour.
Moscow has at various times denied either sending or planning to deploy troops into eastern Ukraine.
The ceasefire deal came one day ahead of a NATO summit where the alliance is to create a "spearhead" rapid reaction force, potentially including several thousand troops, that could be sent to a hot spot in as little as two days, down from an earlier response time of around five days.
On Wednesday, a top Russian defence official said Moscow would adopt a new military doctrine over NATO's expansion and the alliance's plans to establish the rapid-response force in the wake of the rebellion in Ukraine's east.