French President Francois Hollande said a controversial sale of two state-of-the art warships by France to Russia was still on course despite Moscow's widely opposed stand over Crimea.

The 2011 sale of the Mistral warships, worth one billion euros ($1.4 billion), was already a deep source of concern for France's NATO and European Union allies, coming only a few years after Russia's invasion of Georgia.

But despite the threat of another war involving Russia, Hollande, who was in Brussels at a European summit on the Ukrainian crisis, said France's commitment to deliver the military vessels was still alive.

"We respect our signed contracts," Hollande said. "We are not yet at that stage and we hope to avoid getting there," he said, referring to the potential of halting the deal.

The Mistral, the second-largest ship in the French navy, is a high-seas military base that can transport up to 16 helicopters, four landing craft, 13 battle tanks, and hundreds of combat troops.

Experts consider the addition of the Mistral to Russia's ageing fleet as a major leap forward in the Kremlin's ability to act quickly with lethal force.

The first warship, already dubbed the Vladivostok, was floated out in western France last year and is set to be delivered to the Russian navy in October. The second is still under construction.

France and Germany have both risked criticism by pressing the case for dialogue and mediation with Moscow first, downplaying any possibility of sanctions at this stage.

Critics have pointed to lucrative economic deals, such as the Mistral agreement, for the more moderate approach, which differs with the tougher line called for by EU nations in eastern Europe, most former Soviet dependencies.

After six hours of talks Thursday, the European Union's 28 leaders agreed a three-step series of sanctions beginning with the immediate halt of talks on easing visas for Russians as well as discussions over a new economic accord.

British Prime Minister David Cameron said that Britain for its part would look at reviewing its arms sales to Russia.