By Steve Holland and Jeff Mason
RIYADH (Reuters) - U.S. President Barack Obama ended a four-nation foreign trip on Saturday at the same point that he began it, still facing great uncertainty about a diplomatic way out of the Ukraine crisis.
His diplomatic consultations at The Hague, Brussels and Rome over the past week all resulted in a strong show of unity between the United States and Europe that Russia must face consequences should it move against southern or eastern Ukraine.
But whether European allies would be able to stomach the type of crippling sanctions required to undermine the Russian economy in a major way remained an open question, since some of their own economies would be jolted as well.
A late-night phone call on Friday between Obama and Russian President Vladimir Putin offered the possibility that Russia might be willing to negotiate a diplomatic outcome.
But the news was greeted warily by U.S. officials who wondered if Putin really wants to make a deal.
Obama talked to Putin just after meeting Saudi King Abdullah where the civil war in Syria, another major bone of contention between the United States and Russia, was a main topic of conversation.
U.S. officials now will "see whether Russians are serious about diplomacy" on Ukraine, was how one senior Obama administration official described the aftermath of the phone call.
Not lost on them was that the Russian government had assured the West it would make no move against the Crimea region of southern Ukraine. And then it did.
Now, with as many as 40,000 Russian troops massed on Ukraine's border, U.S. officials are increasingly concerned.
In addition, the Russian statement on the Putin-Obama phone call said the Russian president raised concerns about Transnistria, the Russian-majority section of Moldova.
At the heart of subsequent negotiations expected by U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov is a U.S. diplomatic "off-ramp".
In it, international monitors would be deployed to Ukraine to assure ethnic Russians are safe, there would be a pull back of Russian forces, and a direct Russia-Ukraine dialogue.
To some extent U.S. officials are still guessing at Putin's intentions in the region. During a visit to The Hague, Obama said Russia was a "regional power" looking to exert influence in the region.
"I think he's been willing to show a deeply held grievance about what he considers to be the loss of the Soviet Union," Obama told CBS News in an interview on Friday.
"I think there's a strong sense of Russian nationalism and a sense that somehow the West has taken advantage of Russia in the past and that he wants to in some fashion, you know, reverse that or make up for that."
Part of Obama's challenge is not just to convince the Europeans the need for strong action but to persuade Americans at home why they should be interested about what happens in a distant part of the world.
A CBS News poll taken in recent days said 56 percent of Americans approve of sanctions enacted thus far by the United States and European nations, but 65 percent do not think the U.S. should provide military aid and weapons to Ukraine.
In addition, 57 percent said the United States does not have a responsibility to do something about Ukraine.
Obama himself said he could understand why people "might decide to look the other way," but that the "international order" must be protected.
(Editing by Sophie Hares)