The U.S. is defending the sovereignty of Ukraine through applying sanctions against Russia, U.S. President Barack Obama has said.

"We're upholding the principle that bigger nations can't bully the small – by opposing Russian aggression, supporting Ukraine's democracy, and reassuring our NATO allies," Obama said in his state of the union address on Tuesday.

In such a way, the U.S. "demonstrates the power of American strength and diplomacy."

Russia has found itself in isolation with its economy strongly affected by Western sanctions, the U.S. president said.

"Last year, as we were doing the hard work of imposing sanctions along with our allies, some suggested that Mr. Putin's aggression was a masterful display of strategy and strength. Well, today, it is America that stands strong and united with our allies, while Russia is isolated, with its economy in tatters," Obama said.