World leaders and military veterans have gathered in France to mark the 70th anniversary of the Normandy landings, one of the most decisive operations of World War Two.

Dignitaries including US President Barack Obama and the Russian President Vladimir Putin are taking part in the ceremony to pay tribute to the thousands of soldiers who lost their lives in the beach landings in Nazi-occupied France.

Speaking after opening remarks by French President Francois Hollande, Obama told veterans of the operation their "legacy is in good hands".

"Whenever the world makes you cynical,stop and think of these men," he said.

Much of this year's commerations have been overshadowed by the crisis in Ukraine, which has sparked a bitter diplomatic dispute between the US and Russia.

The D-Day invasion was a turning point in the war and paved the way for the liberation of Western Europe from German forces.

More than 150,000 troops parachuted or waded on to French soil on June 6, 1944.

About 5,000 allied soldiers died in the beach invasion and tens of thousands more would be killed in the subsequent battle to free the continent.

Al Jazeera's James Bays reporting from Normandy said with the number of veterans dying due to old age, this year's commemoration would be one of the last attended by a "sizeable number" of soldiers who fought in the war.

Blog: Remembering D-Day 70 years on

In a declaration on Friday, Obama announced that June 6 would be a national remembrance day.

The president praised the soldiers who took part in the invasion for "changing the course on an entire century".

Obama added that the US would "carry on the struggle for liberty and universal human rights," the AP news agency reported.

France's President Francois Hollande is expected to mediate efforts for an 'ice-breaking' meeting between Putin and Ukrainian president-elect Petro Poroshenko.

Putin, who has said he is open to meeting both Obama and Poroshenko while in France, has yet to recognise the legitimacy of the Ukrainian leader who is set to be sworn in on Saturday.

The Russian leader met with German Chancellor Angela Merkel on the sidelines of the events marking the anniversary.

In a guest article for the French newspaper Ouest France, she accused the Russians of demonstrating behaviours that belonged to the nineteenth or twentieth centuries.

At a Group of Seven summit of world leaders in Brussels on Thursday, Hollande called the D-Day tribute "an important occasion to express gratitude and fraternity".