By Roberta Rampton and Jeff Mason

WASHINGTON (Reuters) - President Barack Obama and leaders of five Nordic nations presented a united front against Russia on Friday, expressing concern about Moscow's military buildup in the Baltics and calling for sanctions against Russia to continue.

The leaders of Sweden, Denmark, Finland, Norway and Iceland gathered at the White House for talks that focused on Russia and the crisis in Syria and Iraq that has sparked a flood of migrants to Europe.

Russia's annexation of Ukraine's Crimea region in 2014 alarmed its Nordic and Baltic neighbors. With the North Atlantic Treaty Organization (NATO) considering ways to try to deter further Russian aggression, the White House wants to show support for its northern European allies.

Obama said the six countries were united in their concern about Russia's "aggressive military presence" in the Baltic and Nordic regions. The Baltic countries are Estonia, Latvia and Lithuania.

Last month, a Russian jet fighter intercepted a U.S. Air Force reconnaissance plane flying a routine route over the Baltic Sea, and two Russian jets buzzed a U.S. guided missile destroyer in the sea.

NATO is planning its biggest build-up in eastern Europe since the Cold War to counter what the alliance considers to be a more aggressive Russia. Sweden and Finland are not NATO members.

"We will be maintaining ongoing dialogue and seek cooperation with Russia, but we also want to make sure that we are prepared and strong, and we want to encourage Russia to keep its military activities in full compliance with international obligations," Obama said after meeting with the leaders.

The president is limited by the political calendar in what he can promise, given that his second and final term ends next year on Jan. 20. Americans are set to hold presidential elections on Nov. 8.

The visit will culminate in a star-studded state dinner in a tent with a transparent ceiling, with lighting, flowers and ice sculptures evoking the northern lights.

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Pop star Demi Lovato, known for her support of liberal causes, will perform after guests enjoy a main course of ahi tuna, tomato tartare, and red wine braised beef short ribs.

Obama lauded the humanitarian and environmental accomplishments of his guest nations, which have been key supporters of an international deal to curb climate change that the White House sees as a key part of Obama's legacy.

"There have been times where I've said, why don't we just put all these small countries in charge for a while? And they could clean things up," Obama said.

(Reporting by Roberta Rampton and Jeff Mason in Washington; Additional reporting by Timothy Gardner; Editing by Richard Chang)