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.