British Prime Minister Boris Johnson told Ukrainian President Volodymyr Zelensky during a phone call on Tuesday that Ukraine could retake territory recently seized from Russia, a Downing Street spokeswoman said.

"The prime minister said that the world stands behind Ukraine and he believed that President Zelensky's army can recapture the territory recently captured by Putin's forces," she added, Telegraph reports.

Johnson updated Zelensky on the latest UK military equipment being sent to Ukraine, "including 10 self-propelled artillery systems and cluster munitions, which would arrive in the coming days and weeks", the spokeswoman said.

The two also talked about British citizens who have been seized by the Russians in Ukraine.

They are Aiden Aslin, 28, and Shaun Pinner, 48, who were captured by Russian forces while fighting alongside the Ukrainians.

They have been sentenced to death by pro-Russian separatists, despite Aslin appealing the verdict.