President Barack Obama opens a six-day trip to Europe and Saudi Arabia on Monday seeking to reassure US allies amid the worst East-West crisis in years and to defend 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 it has been transformed into the most important trip to Europe by a US president in years because of the sudden crisis over Russia's annexation of Crimea.

Obama has called a G7 summit in The Hague to deepen his effort to isolate Russia and present it with "costs" over its incursion into the Ukrainian region.

Shockwaves from Crimea's return to Russia will overshadow The Hague summit and provide the subtext to Obama's onward trip to Brussels to meet EU and NATO leaders.

The trip comes as Western leaders rethink their relationship with Russia, following a post-Cold War period in which they sought to usher Moscow into the broader international community.

"I think for Mr Putin, the only thing that has surprised him these past three weeks from Washington was just how weak our response has been," said Andrew Kuchins, of the Center for Strategic and International Studies.

Washington has slapped visa bans and personal financial sanctions on some top officials in President Vladimir Putin's inner circle, but has yet to aim painful blows at the heart of Russia's economy.

One of Obama's core messages for the trip will be that Washington will stand by security guarantees for its NATO partners, including post-Soviet states who joined the alliance.

Obama will also send a message of solidarity to US allies when he sits down with South Korean President Park Geun-Hye and Japanese Prime Minister Shinzo Abe in The Hague.

Washington is keen that historical disputes between the two allies do not disrupt its Asian alliance network at a time of rising tensions between East Asian powers and China.

"If there's a common theme to this trip, it's the fundamental strength and importance of our alliances and partnerships," said National Security Adviser Susan Rice.

"The strategic importance of this effort really can't be overstated. From Europe to Asia to the Middle East, our ability to lead strong coalitions is essential to making progress," she said.

The president will also sit down with Chinese President Xi Jinping in The Hague in their first meeting this year.

- United Front -

Obama and European leaders will seek to project a united front against what they see as a violation of international law committed when Russian troops entered Crimea in an incursion swiftly followed by a referendum to join Russia.

But it is unclear if key EU states, which have greater exposure to Russian trade, investment and energy exports than the United States, will sign up for truly punishing economic sanctions in the event of new provocations from Moscow.

"These sanctions will be very difficult and very painful," said Heather Conley, also of the Center for Strategic and International Studies.

"That is why the United States has to work very hard to convince the most reluctant -- which will be our three strongest allies in Europe: Germany, France and the UK."

In Brussels, Obama will meet EU leaders Herman Van Rompuy and Jose Manuel Barroso, and NATO secretary general Anders Fogh Rasmussen.

It will be Obama's first visit to EU headquarters as president and will come at a time when relations have been tested by the furor over US surveillance operations revealed by fugitive US intelligence contractor Edward Snowden.

On Thursday, Obama will head to the Vatican for an audience with Pope Francis, following his approving recent comments about the pontiff's warnings on economic inequality.

Obama will end his trip in Saudi Arabia, where he will try to cool concerns about his nuclear diplomacy with Iran, which is viewed with skepticism by the government in Riyadh.

"There's a nightmare for the Saudis if Iran gets the bomb, and you can play out that scenario," said George Perkovich of the Carnegie Endowment for International Peace.

"There's also a nightmare if Iran doesn't get the bomb, resolves the issue diplomatically, and comes back into some kind of more normal relationship with the United States."