Prime Minister Boris Johnson vowed Friday that Britain would use all its "diplomatic and economic powers" to counter Russian "aggression" towards Ukraine, in a call with the Ukrainian president.  

The conversation with Volodymyr Zelenskyy follows the UK leader speaking Monday with Russian President Vladimir Putin, when he relayed "deep concern" over Moscow's build-up of forces at its Ukrainian border.

It comes at a time of severely strained ties between Russia and Western capitals, where leaders in recent weeks have accused Moscow of preparing an invasion of its neighbour to the west.

Johnson's office said in a statement following the Friday call with Zelenskyy that the British prime minister had "reiterated the message relayed to President Putin about the UK's support for Ukraine's territorial integrity". 

"The Prime Minister underlined that the UK will use all its diplomatic and economic powers, in concert with allies, to prevent any Russian aggression towards Ukraine," it added.

Downing Street said the pair also "discussed the importance of Ukraine and other European states reducing their dependence on Russian gas and the need to cultivate green and diverse energy sources".

The leaders agreed about the "value of ongoing dialogue, including with Russia", the statement added, noting they would stay in close contact.