Ukrainian President Volodymyr Zelensky on Thursday telephoned Boris Johnson to express his "sadness" over the British leader's resignation as Conservative leader and impending departure as prime minister, Kyiv said.

"We all welcome this news with sadness. Not only me, but also all of Ukrainian society which sympathises with you a lot," the presidency quoted Zelensky as saying, reiterating how grateful Ukrainians were for the British prime minister's support since the Russian invasion.

Johnson has been seen as one of Ukraine's most vocal and fervent supporters in the West.

"We don't doubt that Great Britain's support will continue, but your personal leadership and your charisma made it special," said Zelensky.

The two leaders also discussed military and political cooperation between the two countries and negotiations designed to unblock grain exports from Ukrainian ports, Kyiv added.