President Barack Obama pays his first visit ever to European Union headquarters, cementing US-EU opposition to the takeover of Crimea after hitting out at Russian expansionism as a "sign of weakness".
Viewed as the most important trip to Europe by a US president in years because of the crisis over Crimea, the visit will see Obama hold his first-ever summit talks in Brussels with the EU's top officials, a move seen as an ice-breaker.
"It was time for a summit in Brussels, not in Washington, not anywhere else, but in Brussels," said a senior EU diplomat speaking on condition of anonymity.
Obama, who flew in after a nuclear security summit in The Hague, remains in the EU capital for less than 24 hours before heading for Italy, his next stop on a six-day European tour.
Obama's talks with the presidents of the EU Council and Commission, Herman Van Rompuy and Jose Manuel Barroso -- a midday encounter over lunch scheduled to last 75 minutes -- will be dominated by the crisis in Ukraine.
At a speech he will give later, his only address during the Europe tour, Obama will "step back and look at the current events in Ukraine in a broader context", a senior White House official said.
"Standing at the heart of Europe, in Brussels, the centre of the European project, he'll be able to speak about the importance of European security."
In Crimea, Russian troops stormed the last naval vessel still flying the Ukrainian flag Tuesday, after Ukraine's forces undertook a humiliating withdrawal from the Black Sea peninsula.
In The Hague on Monday, the G7 group of top economic powers in retaliation deepened Moscow's isolation by scrapping a meeting planned in June in Sochi, switching the venue to Brussels and denying Russia an invite.
The Kremlin shrugged off the snub, however.
Obama said the G7 nations were "united in imposing a cost on Russia for its actions", though no decisions were taken to heap further sanctions on Moscow for its actions in Crimea.
Europe's leaders are keen to project a united front against Russia's perceived violation of post-war borders and international law, but EU nations are far more reliant than the United States on Russian trade, investment and energy exports.
In Brussels, Obama will also meet NATO Secretary General Anders Fogh Rasmussen, who last week said Russia's intervention in Ukraine posed the most serious threat to Europe's security since the end of the Cold War.
"This is a wake-up call," he said. "For the Euro-Atlantic community. For NATO. And for all those committed to a Europe whole, free and at peace."
One of Obama's core messages for his Europe trip will be that Washington stands by security guarantees for its NATO partners, including post-Soviet states who joined the alliance.
And as the world economy trundles slowly back to recovery, Brussels and Washington will also pledge new efforts to create the world's largest free trade area, a move that potentially can energise massive growth and create jobs.
A fourth round of talks to conclude the Transatlantic Trade and Investment Partnership (TTIP), worth hundreds of billions to both sides, wound up two weeks ago in Brussels with negotiators claiming progress but no breakthrough.
But at an EU summit last week that was dominated by the Ukraine crisis, the bloc's 28 leaders agreed on urgent steps to reduce Europe's dependency on Russian gas imports, including the possible import of US shale gas as part of the TTIP accord.
Likely to inject a sour note at Wednesday's talks, however, will be lingering controversy over the damaging revelations exposed by fugitive intelligence contractor Edward Snowden of US spying on EU phones.
Obama has promised to reform data collection by US intelligence agencies and an EU diplomat close to the matter reported progress in EU-US cooperation on the matter.
"I don't think this is a subject that will take up much time," he said. "Work is not only ongoing, it is going well."
Obama on Wednesday morning will visit the only US World War I cemetery in Belgium along with Belgian King Philippe and Prime Minister Elio Di Rupo.