The office of Ukrainian President Petro Poroshenko has said that he and his Russian counterpart have reached agreement on a "permanent ceasefire" in eastern Ukraine, however Vladimir Putin's spokesman said the leaders had only discussed "steps" towards a truce.

The separatists rejected the move, saying no ceasefire was possible without Ukraine withdrawing its forces.

"The conversation resulted in an agreement on a lasting ceasefire in Donbas. Mutual understanding was achieved regarding steps that would promote the establishment of peace," the Ukrainian presidency's statement said on Wednesday after Poroshenko and Putin spoke by telephone.

Putin's spokesman, Dmitry Peskov, told the Russian news agency Ria-Novosti that the two leaders had not agreed on a ceasefire because Moscow is not party to the conflict, adding: "They only discussed how to settle the conflict."

"I believe that a final agreement between the authorities of Kiev and southeast Ukraine can be reached and cemented during a meeting of the Contact Group on September 5," Putin said.

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.