NATO Secretary General Jens Stoltenberg says it is "encouraging" to see Ukrainian forces being able to liberate more of the country's territory, after Russian Defence Minister Sergei Shoigu ordered his troops to withdraw from Kherson.
Speaking in Downing Street, where he was meeting British Prime Minister Rishi Sunak, Stoltenberg said: "It is encouraging to see how the brave Ukrainian forces are able to liberate more Ukrainian territory.
"The victories, the gains the Ukrainian armed forces are making belong to the brave, courageous Ukrainian soldiers, but of course the support they receive from the United Kingdom, from NATO allies and partners is also essential," he added.
Asked later during an interview with Sky News about the Kherson retreat, he said: "It's part of an overall pattern or picture we have seen over the last month that Russia has absolutely lost the momentum."
"But we should not underestimate Russia, they still have capabilities," he added. "We have seen the drones, we have seen the missile attacks. It shows that Russia can still inflict a lot of damage."
Following the meeting with Sunak, a Downing Street spokesperson said the pair had emphasised the importance of continuing to support Ukrainian sovereignty.
"(Stoltenberg) thanked the prime minister for the UK's support for Ukraine, which the prime minister confirmed would continue under his leadership for as long as needed," the spokesperson said in a statement.
Earlier, Stoltenberg joined British defence minister Ben Wallace to observe the training of Ukrainian troops in southeast England, as Britain announced the delivery of a further 12,000 extreme cold-weather sleeping kits for Ukraine.