British Prime Minister Theresa May, in a telephone conversation with President-elect of Ukraine Volodymyr Zelensky, has pointed out the need to work together to restrain "Russian aggression," the premier's office said.

"The Prime Minister stressed the importance of our two countries working together alongside the international community to deter Russian aggression," the British PM's Office said in a press release.

She also told Zelensky about her desire to strengthen relations between London and Kyiv.

"She assured President-elect Zelensky of the UK's support for his upcoming Presidency and Ukraine's democratic future, and our willingness to continue and increase our already deep partnership across a range of areas," a Downing Street spokesperson said.

May congratulated Zelensky on his election victory and invited the President-elect of Ukraine to visit the UK "at the earliest opportunity."