Boris Johnson will travel to Kyiv Tuesday for talks with Ukrainian President Volodymyr Zelensky amid mounting tension over Russia’s military build-up at the border, Downing Street announced.

The U.K. prime minister’s visit will take place hours after a planned call between Johnson and Russian President Vladimir Putin, set for Monday afternoon.

Downing Street said Johnson would urge the Russian president to choose diplomacy over military action in Ukraine.

“Our aim is to continue to encourage Russia to take a diplomatic path, to de-escalate and to step back from what could be a very costly exercise if they were to follow the path of further aggression,” the prime minister’s official spokesman said. “He has been clear at all points that pursuing that path would be extremely costly for the Russian people and it’s something we want to avoid and will continue to negotiate on.”

Speaking to reporters Monday, Johnson said Putin needed to “step back from the brink.”

And he insisted “any incursion into Ukraine beyond the territory that Russia has already taken in 2014 would be an absolute disaster for the world, and above all it would be a disaster for Russia.”

Johnson’s foreign secretary, Liz Truss, will make a statement in the House of Commons later unveiling new legislation to widen the range of Russian individuals and businesses it can target for sanctions in the event of an incursion into Ukraine.

The U.K. announced this weekend it is considering making the “biggest possible offer to NATO,” including sending defensive weapons to nearby Estonia and doubling troop levels there.