Ukraine crisis: France halts warship delivery to Russia
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, saying it had to respect an existing contract.
Russia's Deputy Defence Minister Yury Borisov said the French decision would not hold back Moscow's plans to reform its armed forces.
"Although of course it is unpleasant and adds to certain tensions in relations with our French partners, the cancelling of this contract will not be a tragedy for our modernisation," he said, quoted by Itar-Tass news agency.
Ukrainian Foreign Minister Pavlo Klimkin tweeted his thanks to the French leadership for its "responsible decision", which he said was "important for restoring peace in Europe".
The Vladivostok, the first of the two helicopter carriers, was expected to have been delivered to Russia by late October.
The second, the Sevastopol, was to have been sent next year, although no mention of it was made in Mr Hollande's statement.
Analysis: Jonathan Marcus, BBC defence and diplomatic correspondent
As the crisis has escalated in eastern Ukraine and as Russia's direct military role there has become more blatant, so the pressure on the French government to halt its sale of two advanced assault ships to Russia has grown ever stronger.
The US and a number of other countries have long made their feelings plain. But the deal weathered tensions with Moscow over Syria, and the Russian crew of the first vessel which is already undergoing sea trials has travelled to France to begin training.
This was the most significant Western arms sale to Russia and its postponement - the exact terms of the suspension of the deal are not clear - marks a very visible rebuff to Moscow on the eve of Nato's Wales Summit.
The Mistral assault ships can carry up to 16 heavy helicopters, land troops and armoured vehicles. Their delivery would have resulted in a marked improvement in Russia's amphibious capability.
But Mr 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.
Previously France had argued that EU sanctions could not apply retroactively to the Mistral contract, and that it would have been too costly to cancel.
The deal for the two helicopter carriers is worth 1.2bn euros ($1.6bn; £0.95bn).
Separately Poland has given details of military exercises to be held later this month in Ukraine.
A defence ministry statement said the exercises, codenamed Rapid Trident 14, will involve hundreds of soldiers from Nato member states including the US.
Peace deal hopes
Earlier on Wednesday, Ukrainian President Petro Poroshenko said he had agreed a "ceasefire process" with his Russian counterpart Vladimir Putin.
Mr Putin said he hoped a peace deal could be reached by Friday, when representatives of Russia, Ukraine and the rebels meet in Minsk for talks.
The pro-Russian rebels have said they support Mr Putin's proposals, but that they do not trust Mr Poroshenko to maintain a ceasefire.
It is not clear whether any truce is being observed on the ground.
Meanwhile, in Estonia, US President Barack Obama sought to reassure the Baltic states that they would be protected by Nato, and said that Washington would stand by Ukraine.
Analysis: Oleg Boldyrev, BBC News, Moscow
Insisting that Russia is in no way a negotiating party in the Ukrainian conflict, Vladimir Putin is nevertheless certain that it is Moscow's proposals that are going to advance both sides to peace. Mr Putin's plan is short and leaves a lot of room for interpretation.
Take, for example, a point on moving Ukrainian troops away from positions from which towns and cities can be shelled. Taken to an extreme, this could mean rewinding the situation on the ground to a point several weeks ago before Ukrainian advances.
Just two weeks ago the discussions centred around when, and not if, the rebels would have to move out of Donetsk and Luhansk. But Moscow's plan will allow them to strengthen their control over the two regional centres and other areas.
Mr Putin has often seemed unwilling to negotiate from a position of weakness and the reversals of the past few days illustrate this perfectly. Now it's Petro Poroshenko who has to choose whether to accept something which clearly protects Kiev's enemies in eastern Ukraine.
The announcements come 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.
More than 2,600 civilians and combatants have been killed and more than a million people have fled their homes since fighting erupted in eastern Ukraine in April, when pro-Russian separatists there declared independence.
Russia has denied accusations by the West and the Ukrainian government that it is sending troops and military equipment over the border to support the separatists, who recently gained the upper hand against government forces.