President Obama talked informally to Russian President Vladimir Putin at a lunch to celebrate D-Day in France, urging him to take advantage of an opportunity to recognize the election of Ukraine's president as a way to de-escalate the crisis in Ukraine.

A senior administration official said Obama left the leaders lunch after his conversation with Putin feeling "more optimistic" about the likelihood that Russia would take steps to ease tensions in Ukraine.

“This is a natural point in time that you’re not going to get again,” the official said, referring to the election. “It provided for a new basis of legitimacy for the government. It provides a new interlocutor” for Putin.

“There is a window open here," the official added.

The pair spoke for 10 to 15 minutes on the sidelines of the Friday afternoon lunch, Deputy National Security Adviser Ben Rhodes told reporters traveling with Obama.

Reporters were keeping a close eye on interactions between Obama and Putin during the set up for the meal and a group photograph of the leaders gathered for the commemoration event at a chateau in Benouville, France.

At times, Obama and Putin were close enough to touch but did not speak with or acknowledge each other while reporters were watching. Obama was seen jovially greeting other leaders, kissing German Chancellor Angela Merkel on both cheeks, but appearing to steer clear of any direct encounter with Putin.

At the end of the lunch, as the leaders were standing Obama and Putin struck up a conversation at the table, according to a senior administration official who declined to say who initiated the talk or whether an intermediary started the conversation.

“It was something they expected, that they’d speak with each other,” the official said.

The White House said Obama told Putin Ukraine's President-elect Petro Poroshenko's election provides an opportunity for Russia to recognize him as the legitimate leader of Ukraine and to cease its support for separatists in eastern Ukraine, including the supply of arms and other materials across the border, the White House said in a read-out of the discussion.

Obama also stressed Poroshenko's commitment to pursue reforms and to ensure that the rights and interests of all Ukrainians are respected.

The White House did not say how Putin responded.

Putin and Ukraine's President-elect Petro Poroshenko also talked briefly before leaders went into the lunch while Merkel stood between them, but reporters could not hear what was said. The possibility of a Putin-Poroshenko meeting was discussed during the G-7 meeting in France on Friday and French President Francois Hollande followed through by arrange it.

During the short encounter, the two broadly discussed the notion of a ceasefire, according to the Obama administration official, who said Poroshenko emphasized that it had to be a two-way street with Russia agreeing to work to sop the violence, not just Ukraine.

The Putin-Poroshenko meeting was a "positive development," the official said, but not a substitute for Putin formally recognizing Poroshenko's election or taking other steps Obama and European leaders have outlined to de-escalate the situation.

The diplomatic tensions between the United States and Russia that arose after the crisis in Ukraine forced Hollande to hold two separate successive dinners Thursday night - the first with Obama at a restaurant off the Champs-Elysees and a second, two hours later, at the presidential palace with Putin.