U.S. President Joe Biden will meet with his counterparts from the Group of Seven allies early Thursday to map out more severe measures against Russia after President Vladimir Putin launched what Biden called "a premeditated war" against Ukraine.

Biden, who spoke with Ukrainian President Volodymyr Zelenskiy late on Wednesday, said the United States and its allies will respond in "a united and decisive way" to what he called "an unprovoked and unjustified attack by Russian military forces" on Ukraine.

"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.

It came shortly after Putin told Russian state TV he had authorised a military operation in eastern Ukraine and explosions were heard in the Ukrainian capital of Kyiv and the breakaway eastern Ukrainian city of Donetsk..

The White House said Biden was briefed late Wednesday by Secretary of State Antony Blinken, Defense Secretary Lloyd Austin and other top advisers. He planned to deliver remarks in the early afternoon to announce "the further consequences the United States and our allies and partners will impose on Russia," the White House said.

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

Biden, who served as America's vice president and was deeply engaged in 2014 when Russia annexed the Crimea region of Ukraine, will be joined for the G7 call in the White House Situation Room by Blinken and Treasury Secretary Janet Yellen.

TOUGHER SANCTIONS URGED

Biden, who orchestrated an initial round of Western sanctions against Russian oligarchs, financial institutions and exports this week, is under pressure from fellow Democrats and Republicans in Congress to crack down even harder on Moscow.

House Intelligence Committee Chairman Adam Schiff, a fellow Democrat, told CNN on Wednesday that Western actions were being closely watched by China with an eye to whether it could successfully invade Taiwan.

"We have to make sure that if Putin goes forward with this invasion more fully ... that the costs to Putin and Russia are just crippling," he said.

Schiff said the United States and its allies should put in place "the most severe sanctions as soon as possible," including a permanent end to the Nord Stream 2 pipeline connecting Russia with Germany under the Baltic Sea.

Republican Senator Rob Portman demanded tough sanctions, rigid export controls and moves to increase military support to Ukraine and other allies, including Poland, Romania and the Baltic countries, in a statement after the Russian attacks.

U.S. officials this week have warned that Moscow will pay an even steeper price if it continues its aggression.

Washington on Wednesday stepped up pressure on Putin by imposing sanctions on the firm building the $11 billion Nord Stream 2 gas pipeline and its corporate officers, a move Biden had waived for months.

Germany on Tuesday froze approvals for the pipeline, which has been built but was not yet in operation, amid concerns it could allow Moscow to weaponize energy supplies to Europe.

Next steps are likely to include sanctions against additional Russian banks, including Sberbank and VTB, steps to bar U.S. financial institutions from processing transactions for major Russian banks, and export controls on U.S. and foreign-made goods, from commercial electronics and computers to semiconductors and aircraft parts.