WASHINGTON: The United States and its allies will respond in a united and decisive way to "an unprovoked and unjustified attack by Russian military forces" on Ukraine, U.S. President Joe Biden said after blasts were heard in the Ukrainian capital of Kyiv.

"President (Vladimir) Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering," Biden said in a statement issued late on Wednesday.

"Russia alone is responsible for the death and destruction this attack will bring. The world will hold Russia accountable."

Biden said he would announce on Thursday further measures to be imposed on Russia by the United States and its allies.

The White House said Biden would speak in the early afternoon.

Biden's statement came shortly after Putin told Russian state TV he had authorized a military operation in eastern Ukraine and explosions were heard in the Ukrainian capital of Kyiv and the breakaway eastern Ukrainian city of Donetsk. Read full story

Biden said he would monitor the situation from the White House before meeting virtually with his counterparts from the Group of Seven advanced economies early on Thursday.

Then, he said, he would "speak to the American people to announce the further consequences the United States and our allies and partners will impose on Russia for this needless act of aggression against Ukraine and global peace and security."

Biden said Washington would also coordinate with NATO allies "to ensure a strong, united response that deters any aggression against the alliance."