Former British Prime Minister Boris Johnson visited Kyiv on a surprise trip on Sunday, meeting President Volodymyr Zelenskiy and pledging that Britain would "stick by Ukraine as long as it takes."

Johnson, who left office in September in the wake of a series of scandals, was prime minister when Russia invaded Ukraine last February and he sought to position London as Kyiv's top ally in the West.

During his trip, Johnson visited Borodyanka and Bucha, the suburbs of the Ukrainian capital that became a byword for atrocities in the West when Russian forces drove towards Kyiv in the first phase of the invasion before being repelled.

"I can tell you that the UK will be sticking by Ukraine for as long as it takes," Johnson told the mayor of Bucha.

"You're going to win and you're going to get all the Russians out of your country, but we will be there for the long-term. And we will also want to be helping you to reconstruct," he said speaking through an interpreter.

Johnson has dismissed suggestions that his activity in Ukraine could be seen as undermining British Prime Minister Rusni Sunak.

While in office, Johnson visited Kyiv several times and called Zelenskiy frequently.

As he became embroiled in scandals in Britain, he gained widespread popularity in Ukraine where he became known affectionately as "Borys Johnsoniuk." Cafes named cakes after him in Kyiv and street art was produced using his image.

In Bucha, Johnson took selfies with residents and laid flowers in tribute to victims of the war. He visited a church to see an exhibition and signed the Ukrainian edition of his book about Winston Churchill for a priest.

In Borodyanka, he walked the streets by ruined residential blocks. Kyiv regional governor Oleksiy Kuleba, who accompanied him, said that 162 residents were killed during the month-long Russian occupation of the town last year. Kuleba said about 60% of residents have since returned.

In Kyiv, Johnson was personally welcomed by Zelenskiy and a row of top officials including the foreign minister and the head of the president's office who lined up in a yard near the presidential administration in the heart of the city.

Last week Britain said it would supply Ukraine with 14 Challenger 2 tanks and other heavy weaponry.