What has President Putin claimed about Russian troops in Ukraine's Sumy region and how has Ukraine responded?

Russian President Vladimir Putin has announced that his troops have crossed into Ukraine's northeastern Sumy region, marking what would be the first ground attack there since 2022. This claim represents a potential significant escalation in the ongoing conflict between Russia and Ukraine. However, the Ukrainian government in Kyiv has swiftly denied Putin's assertion. Ukrainian officials have refuted Moscow's claims of launching a major military operation in the Sumy region, maintaining that no such incursion has occurred as claimed by the Russian president.