France has said conditions are “not right” for delivery of the first of two Mistral navy assault ships to Russia.
President Francois Hollande’s office blamed Moscow’s recent actions in Ukraine.
France had until now resisted pressure to halt the delivery.
Correspondents say the EU has struggled to tighten sanctions against Russia, restricting itself to travel bans and asset freezes against individuals. New sanctions are expected within days.
The Vladivostok, the first of the two helicopter carriers, was expected to have been delivered to Russia by late October.
But Hollande’s office said Wednesday’s remarks by the Russian and Ukrainian presidents about a possible ceasefire were not enough to allow France to give it the go-ahead.
“The president of the republic has concluded that despite the prospect of ceasefire, which has yet to be confirmed and put in place, the conditions under which France could authorise the delivery of the first helicopter carrier are not in place,” it said in a statement.
The announcement comes a day before a Nato summit in Wales which is expected to discuss the alliance’s response to Russia’s involvement in the Ukraine conflict.