By Max Hunder

KYIV (Reuters) -Russia has massed more than 50,000 troops, including some of its best forces, near Ukraine's northeastern Sumy region, but Kyiv has taken steps to prevent them from conducting a large-scale offensive, Ukrainian President Volodymyr Zelenskiy said.

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 small areas there.

"Their largest, strongest forces are currently on the Kursk front," Zelenskiy 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. Zelenskiy said he believed Russia wants to carve out an area of Ukrainian territory about 10 km (6 miles) deep.

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 Kostiantynivka.

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

Zelenskiy told reporters in a briefing that his government was ready for further peace talks in any format. He said he expected the next round to be at a technical level, but said he would be ready for a three-way meeting with U.S. President Donald Trump and Russian President Vladimir Putin.

He said he did not want the United States to leave the Ukraine peace process, as Washington has threatened to do if progress is not made.

VENUE FOR TALKS

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.

Zelenskiy said 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.

Zelenskiy said Russia had signalled that it wanted talks to take place in Belarus, a suggestion Kyiv regarded as unserious and impossible given Russian troops used the country as a launchpad to invade Ukraine in 2022.

He also 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, Zelenskiy said he wanted $30 billion for Ukraine this year to fully fund the available capacity of the rapidly expanding sector.

(Reporting by Max Hunder; Editing by Raju Gopalakrishnan and Ros Russell)