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British Prime Minister Boris Johnson will speak to U.S. President Joe Biden and other world leaders later on Tuesday to discuss toughening sanctions against Russia and further support for Ukraine, Johnson's spokesman said.

Leaders from Canada, France, Germany, Italy, Romania, Japan, NATO and the European Commission are also due to join the call.

Earlier, a senior national security official gave Johnson and his ministers an update on Britain's assessment of the situation in Ukraine, the spokesman said.

"The next phase of the war was likely to be an attritional conflict which could last several months. Russia would aim to exploit its troop number advantage but Ukraine had already shown that this was unlikely to be decisive on its own," the spokesman said, relaying the unnamed official's assessment to reporters.

"There were some signs that Russia had not learned lessons from previous setbacks in northern Ukraine, and there was evidence of troops being committed to the fight in a piecemeal fashion." (Reuters)