A Russian invasion of Ukraine would end in a humanitarian, political and military disaster for Russia and the world, Boris Johnson has warned as he stood alongside the Ukrainian president, Volodymyr Zelenskiy, in Kyiv, saying the UK would be judged by the level of help it gave to Ukraine.

On a flying visit to the Ukrainian capital, he denied the US and the UK were exaggerating the scale of the Russian threat, saying they were not trying to “big up” the intelligence. “The grim reality” was that Russian troops were “massing on Ukraine’s border. This is a clear and present danger,” he said, adding that the troop concentration was “perhaps the biggest demonstration of hostility to Ukraine in our lifetimes”. Johnson said it dwarfed the Russian forces mounted before the invasion in 2014.

The British prime minister said by holding a gun to the head of the Ukrainian people Vladimir Putin was trying to get the west to dismantle the new security architecture set up after the fall of the Berlin Wall. He said the UK was trying to bring the west together, saying this crisis is about something bigger than Ukraine. “He [Putin] is trying to redraw the security map of Europe and to impose a new Yalta, new zones of influence. It would not just be Ukraine that was brought back into the Russian zone of influence. It would be Georgia and Moldova and other countries. This is absolutely critical, this moment.”

The press conference came after Putin, speaking in Moscow, accused the US of ignoring Russia’s security proposals in his first public comments on the growing crisis over Ukraine since December.

A phone call between Johnson and the Russian president that the prime minister was forced to cancel on Monday has been rescheduled for Wednesday, after the Kremlin rejected a request to hold it on Tuesday. The Monday call was cancelled so Johnson could make a statement to MPs in parliament about Downing Street parties during lockdown.

Johnson was dogged by questions at the press conference about whether his domestic political crisis over the parties was making it impossible for him to focus on the Russian crisis. He insisted he was fully focused and said he would be talking to Putin on Wednesday. Their scheduled call had to be cancelled on Monday as Johnson was forced instead to spend two hours answering questions about the allegations of parties in Downing Street in breach of Covid regulations laid out in the report written by the civil servant Sue Gray.