The Kremlin in Moscow has taken a swipe at outgoing UK Prime Minister Boris Johnson, who has overseen consistent British support for Ukraine in its fight against Russia's invasion.
President Putin's spokesman Dmitry Peskov said Mr Johnson "really does not like us - and we [do not like] him either".
He said he hoped "more professional people" who could "make decisions through dialogue" would take over in London.
Meanwhile, Russian foreign ministry spokeswoman Maria Zakharova told reporters Mr Johnson had been "hit by a boomerang launched by himself", adding that the moral of the story was "do not seek to destroy Russia".
But Ukraine struck the opposite tone: the presidency thanked the politician for his support in "the most difficult of times", according to news agency AFP.
President Volodymyr Zelensky called the prime minister on Thursday after he announced his resignation, during which Kyiv officials says the Ukrainian leader expressed his "sadness" upon hearing the news.
"Not only me, but also all of Ukrainian society which sympathises with you a lot," he is reported to have said - thanking him for the "decisive action" he has taken to help Ukraine.
The pair have developed a close relationship since the start of the war. Kyiv's foreign minister, Dmytro Kuleba, also praised the UK's outgoing prime minister in a statement: "We will always remember his visit to Ukraine in the still dark hour of April. Johnson is a man of no fear, ready to take risks for the cause he believes in."
In Washington, President Joe Biden praised the strength and endurance of the "special relationship" between the US and UK, avoiding any reference to Boris Johnson himself or his legacy.
"I look forward to continuing our close co-operation with the government of the United Kingdom, as well as our allies and partners around the world, on a range of important priorities," the US president added.
The Russian officials were not the only critics, however, with a number noting the strained relations Mr Johnson had had at times with international partners following the UK's vote to leave the European Union, which he championed and eventually saw through.
Guy Verhofstadt, the European parliament's former Brexit coordinator, said "EU-UK relations suffered hugely with Johnson's choice of Brexit", adding his reign was ending in "disgrace, just like his friend Donald Trump".
Michel Barnier, the EU's former chief negotiator, said Mr Johnson's departure "opens a new page in relations with" the UK - one he hoped would be "more constructive, more respectful of commitments made, in particular regarding peace & stability in Northern Ireland, and more friendly".
Irish Prime Minister Micheal Martin also saw the resignation as a chance for a reset in relations with the UK. He acknowledged in an official statement that he "didn't always agree" with Mr Johnson, saying relations between the governments had been "strained and challenged in recent times".
"We have now an opportunity to return to the true spirit of partnership and mutual respect that is needed to underpin the gains of the Good Friday Agreement."