French President Francois Hollande urged world leaders in Paris on Thursday for the 70th anniversary of  D-Day to use the occasion to ease tensions over the Ukraine crisis.

Hollande, criticized at home for a lack of initiative on Ukraine, had a working dinner with U.S. President Barack Obama before dashing across Paris to join Russian President Vladimir Putin for a second evening meal.

Hollande and Obama discussed "how to use opportunities in the coming days to bring about a de-escalation," a member of Hollande's entourage said following the meal.

One of Hollande's main aims, diplomats said, is to clear the path for an ice-breaking first meeting on French soil on Friday between Putin and Ukrainian President-elect Petro Poroshenko, even as fighting in eastern Ukraine continues between government forces and pro-Russian separatists.

The separate meals showed the lengths to which French officials have gone to keep Obama and Putin apart in Paris, at Washington's request, before Friday's commemorations of the allied D-Day landings that helped end World War II, which will see 18 world leaders assemble on the Normandy beaches.

The French capital went into security lockdown before Britain's Queen Elizabeth II arrived by train from London. She was followed by Obama, who flew in from a Group of Seven meeting in Brussels to which Putin had not been invited.

"This is an important occasion to express gratitude and fraternity, but it is also major international event which should serve the interests of peace," Hollande told the G7 summit before he, too, rushed back to Paris.

Asked if a Putin-Poroshenko meeting in France was possible, Hollande replied: "Yes ... I invited (Poroshenko) so he could be there because he represents the Ukrainian people who suffered a lot during World War II. He will be there alongside President Putin. President Putin was informed of that."

Obama called on Putin to recognize and work with Ukraine's new government and stop "provocations" along its border, or face tougher sanctions from members of the G7.

Putin said in French media interviews on Wednesday he was open to meetings with both Poroshenko and Obama in France.

But underscoring the sensitivities, Hollande rushed from meeting Queen Elizabeth at the Elysee Palace to a chic restaurant overlooking the Champs-Elysees to dine with Obama before returning to his residence for a late supper with Putin.

Hollande has spent much of the week trying to create a diplomatic opening on Ukraine after sending Poroshenko a last-minute invitation to the D-Day ceremonies. Ukrainians fought in the Soviet Red Army that defeated Nazi Germany.

French diplomats say Hollande, who met Poroshenko in Poland on Wednesday, wants at the very least to get Putin and the Ukrainian to shake hands at a closed-door lunch of leaders on Friday at the 18th-century Chateau de Benouville.

This, they argue, would be a tacit acknowledgement that the Russian leader recognizes Poroshenko's legitimacy, the day before he is sworn in, opening the door for dialogue.

In an apparent signal of recognition, Russia's deputy foreign minister, Grigory Karasin, said Moscow's ambassador to Ukraine would attend Poroshenko's inauguration.