President Barack Obama warned Russia on Tuesday that US support for Ukraine was a show of American strength and diplomacy which had left Moscow isolated and its economy in tatters.

"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," he told US lawmakers in his State of the Union address.
His words came as Kiev accused Russian forces of attacking its soldiers after crossing over into the ex-Soviet state's war-wrecked separatist east in violation of a September truce deal.

The charges -- met with initial silence by Moscow -- could add further tensions to difficult talks in Berlin on Wednesday aimed at stemming a spike in fighting that has already killed 4,800 people and driven a million from their homes.
Obama said last year critics had suggested that Russian President Vladimir 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.

"That's how America leads -- not with bluster, but with persistent, steady resolve."