A week of diplomacy by U.S. President Joe Biden failed to defuse tensions between Russia and Ukraine, but has maintained unity with European allies against Moscow as Russian President Vladimir Putin seeks to expose cracks, foreign policy analysts say.

After former U.S. President Donald Trump openly disparaged NATO and left Europe questioning Washington's commitment to the alliance, some analysts said Putin appeared to be testing Biden's declaration that "America is back" on the world stage.

"He's probably counting on divisions in the West," veteran former U.S. diplomat Daniel Fried, who worked on the response to Russia's seizure of Crimea in 2014, said of Putin. "He's going to keep pushing and things could get worse. But had we caved (this week), it would be much worse."