Speaking with Ukraine’s president, the UK prime minister also acknowledged the “immense suffering being inflicted on civilians”, Downing Street said.
Ukraine says its military has retaken the entire region around the capital, as Russia has withdrawn from key towns.
Mr Johnson also updated Mr Zelensky on a recent military donor conference convened by the UK with 35 countries.
Speaking over the phone with the prime minister, the Ukrainian president gave an update on the steps towards peace negotiations and welcomed “further UK involvement in these diplomatic efforts”, Downing Street said.
Among the areas abandoned by Russia around the capital is Hostomel airport, as well as key battleground towns around the capital of Irpin and Bucha.
As Moscow’s forces retreat, evidence of civilian killings is mounting. In Bucha, a street has been found with at least 20 dead bodies of men in civilian clothes.
A Downing Street spokesperson said Mr Johnson “congratulated Ukraine’s brave armed forces” for their success in pushing back the Russian army in some areas.
Both leaders agreed on the importance of “continuing to ratchet up sanctions to increase the economic pressure” on Russian president Vladimir Putin, as long as Russian troops remained on Ukrainian territory.
Mr Johnson also updated Mr Zelensky on the progress made at a military donor conference and “committed to continue to step up defensive support”, the spokesperson added.
Writing on Twitter, Mr Zelensky described the UK as a “powerful ally” and added: “We look forward to the donors’ conference for Ukraine.”