Pro-Russian rebels shot down a Ukrainian military helicopter killing all nine people on board near Sloviansk.
The Ukrainian army says the Mi-8 helicopter, used for transporting military cargo, was hit by a rocket shortly after take-off outside the rebel-held city of Sloviansk.
It comes a day after the rebels vowed to observe a ceasefire until Friday, in response to a government peace plan.
Russian President Vladimir Putin warned that the week-long truce was not enough.
Vladimir Putin said it should be extended to try to hold “substantive talks” between the Ukrainian government and the separatists.
The rebels have not commented publicly on the Ukrainian military’s claims.
Before Tuesday, the rebels – who continue to hold towns in the regions of Donetsk and Luhansk – had shot down at least two Ukrainian army helicopters and a plane.
Earlier in the day, President Vladimir Putin asked the Russian parliament to revoke the right of military intervention in Ukraine.