No Russia on Obama's Europe trip

US President Barack Obama has left for Europe on a trip aimed at further isolating Russia over its annexation of Crimea and at defending his nuclear diplomacy with Iran.

A visit to the third Nuclear Security Summit in The Hague - which grew out of Obama's initiative to prevent the spread of weapons of mass destruction and radioactive material - has been planned for months.

But as Ukraine voices fears of an imminent Russian invasion, Obama will meet with G7 leaders, confer with European Union leadership and with NATO, where he will seek to send a message that Russia must pay a price for the Crimean crisis.

The G7 meeting, set for Monday in the Dutch city, is a symbolic snub to Moscow, host of a G8 summit this year in Sochi which now looks unlikely to take place.

The trip also includes stops in Brussels, Vatican City and Saudi Arabia, and a meeting with Chinese President Xi Jinping to try to ease tensions between key Asian allies South Korea and Japan.

Obama's meeting with Xi and trilateral summit with South Korean President Park Geun-hye and Japanese Prime Minister Shinzo Abe will highlight his policy of rebalancing US resources towards Asia, which he will visit in April.

Washington is worried about acrimony between Japan and South Korea over historic disputes linked to World War II, at a time of rising regional tensions and territorial disputes.

In the final leg of Obama's trip, in Saudi Arabia, he will meet King Abdullah in an apparent attempt to ease scepticism about his nuclear diplomacy with Iran.

The US's hopes for a summit between Obama and the leaders of Gulf allies were put on hold owing to tensions between regional states and Qatar.