The advance may be linked to Moscow's attempts to create "buffer zones" along the border, Ukrainian regional authorities say.

The warnings over the offensive, come as a U.S.-brokered peace process has limped along with inconclusive meetings and little concrete results.

By Tom Balmforth and Yuliia Dysa KYIV (Reuters) -Russian forces have captured four villages in Ukraine's northeastern Sumy region, the local governor said, the latest battlefield setback for Kyiv as it seeks to hold territory and avoid handing Moscow the advantage in any peace talks.

By Steve Holland, Tom Balmforth and Yuliia Dysa WASHINGTON/KYIV (Reuters) -U.S. President Donald Trump on Tuesday said Vladimir Putin was "playing with fire" by refusing to engage in ceasefire talks with Kyiv as Russian forces made gains in Ukraine's northeast.

President Donald Trump could move ahead with new sanctions on Russia in the coming days as he vents frustration at Russian President Vladimir Putin for his aerial assault on Ukraine over the weekend,

President Trump says that the Russian president has “gone absolutely CRAZY’’ with attacks in Ukraine, but has so far refused to join Europe with its newest sanctions.

Russia and Ukraine swapped hundreds of prisoners Sunday just hours after Moscow launched one of the largest aerial assaults on Ukraine in the ongoing war.

President Zelenskyy denounces what he calls U.S. silence as Russia launches a large-scale assault on Ukraine with drones and missiles.

Russia and Ukraine swapped thousands of prisoners, even as Russia continued a massive bombardment of Ukraine over the weekend. Haley Ott reports.