President Barack Obama underlined US commitment to its NATO allies on Wednesday, showing solidarity with eastern European nations after Russia's intervention in Ukraine.

"As allies, we have a solemn duty - a binding treaty obligation - to defend your territorial integrity. And we will," Obama said, according to the advance text of a speech in Warsaw.

"Poland will never stand alone. Estonia will never stand alone. Latvia will never stand alone. Lithuania will never stand alone. Romania will never stand alone."