Zelenskyy says Russia has gathered 50,000 troops for offensive on northern Ukraine

Kyiv: Ukrainian President Volodymyr Zelenskyy has said that Russia has gathered 50,000 troops near Ukraine's northern Sumy region, but added that Kyiv had taken steps to prevent Moscow from conducting a large-scale offensive there.

The build-up comes as Russia appears to be gearing up for a summer offensive in Ukraine while Kyiv waits for Moscow to present a memorandum laying out its conditions to proceed with ceasefire talks.

Sumy lies across the border from Russia's Kursk region where Ukraine previously seized and held a pocket of land for months, before being almost fully pushed out last month, although it says it still holds some areas there.

"Their largest, strongest forces are currently on the Kursk front," Zelenskyy told reporters on Tuesday. "To push our troops out of the Kursk region and to prepare offensive actions against the Sumy region."

Putin has said he wants a "buffer zone" along Russia's border with Ukraine. Zelenskyy said he believed Russia wanted a buffer zone of about 10 kilometres (6 miles).

Russia has captured at least four border villages in the region recently, and has been creeping slowly forwards over the past several weeks on parts of the frontline in eastern Ukraine near the city of Kostyantynivka.

However, Zelenskyy said that the Russians had been pushed back in that area by 4 km (2.5 miles) over two days.

Ukraine and Russia swapped 1,000 captives each after a meeting of the two countries' delegations in Istanbul nearly two weeks ago which failed to produce a ceasefire sought by Ukraine, the U.S. and Europe.

Zelenskyy said that he viewed Turkey, the Vatican and Switzerland as the most realistic venues for further negotiations with Russia. He said interest in hosting talks had also been expressed by Malta, as well as unspecified African nations.

Reuters previously reported that Moscow does not see the Vatican as a serious venue for talks.

Zelenskyy said that he will attend the next G7 summit after being invited by Canadian Prime Minister Mark Carney, whose country currently holds the bloc's rotating presidency. He added that he will likely take part in the next EU summit.

Speaking about Ukraine's domestic arms production, Zelenskyy said he wanted $30 billion for Ukraine to fully fund the available capacity of the rapidly expanding sector.