COLLEVILLE-SUR-MER, FRANCE - World leaders and veterans gather by the beaches of Normandy on Friday to mark the 70th anniversary of the Allied D-Day landings that helped turn the tables in World War II , with host France hoping the event will bring a thaw in the Ukraine crisis.Wreaths, parades, parachute-landings and fireworks will be staged in honor of history's largest amphibian assault on June 6, 1944 when 160,000 US, British and Canadian troops waded ashore to confront Nazi Germany's forces, hastening its defeat. French President Francois Hollande will be joined at the commemorations by 17 leaders including US President Barack Obama, Britain's David Cameron, Canada's Stephen Harper, Germany's Angela Merkel and Vladimir Putin of Russia.But while the unity of allies and their bloody sacrifices will be the big theme of the D-Day remembrance, the government leaders will be sounding each other out in private on the worst security challenge in Europe since the Cold War: Ukraine.Russia's annexation of Crimea in March and the current standoff in eastern Ukraine between government forces and pro-Russian separatists have driven Russia's relations with the United States and European Union to a post-Cold War low.French diplomats say Hollande hopes to get Putin to at least shake the hand of Ukrainian president-elect Petro Poroshenko on the sidelines of the ceremonies, in what could represent a first step in defusing tensions.Putin, who has said he is open to meeting both Obama and Poroshenko while in France, has yet to recognize the legitimacy of the Ukrainian leader who is set to be sworn in on Saturday.At a Group of Seven (G7) summit of world leaders in Brussels on Thursday, Hollande called the D-Day tribute "an important occasion to express gratitude and fraternity.SUMMIT SNUB"But it is also a major international event which should serve the interests of peace," Hollande added, evoking the diplomatic challenges under the surface of the ceremonies.The G7 summit - to which Putin was not invited because of Western anger over Russia's intervention in Ukraine - was intended "to speak with a single voice ... above all on Ukraine," Hollande said.Obama told reporters on Thursday that Russia would face new sanctions if it fails to recognize Ukraine's new government and does not try to calm violence from militants in the east of Russia's fellow former Soviet neighbour."There is a path in which Russia has the capacity to engage directly with President Poroshenko now. He should take it," Obama said. "If he does not - if he continues a strategy of undermining the sovereignty of Ukraine, then we have no choice but to respond."Further complicating the diplomatic meetings on Friday is US opposition to a 1.2 billion euro ($1.63 billion) French contract to sell two Mistral helicopter carriers to Russia.The US government says the deal sends the wrong message to Russia at a time of economic sanctions imposed by Western states on Moscow over the conflict in Ukraine.Putin's relations with Ukraine as well as with the European Union and the United States have been tense since pro-Western protesters ousted a Moscow-friendly Ukrainian president from power in February and Russia then seized Crimea.Obama and Hollande dined together in Paris on Thursday evening and discussed ways of easing the Ukraine crisis before Hollande held a second, separate dinner with Putin.The separate meals showed the lengths to which French officials have gone to keep the estranged Obama and Putin apart in Paris, at US request, before the D-Day commemorations.