Russian President Vladimir Putin is making "slow but tangible progress in Donbass in eastern Ukraine, said British Prime Minister Boris Johnson.

"I'm afraid that Putin, at great cost to himself and to the Russian military, is continuing to chew through ground in Donbas," he told Bloomberg UK.

"He's continuing to make gradual, slow, but I'm afraid palpable, progress and therefore it is absolutely vital that we continue to support the Ukrainians militarily."

According to him, Ukraine needs additional military support, including multiple launch rocket systems.

"The one way that it can end is for Putin to accept that ... the denazification of Ukraine has taken place and he's able to withdraw with dignity and honour," he said, adding this did not mean he supported Russia's premise that there were Nazis in Ukraine.