Trump Putin Talks Collapse /Lt Col Daniel Davis

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Dates haven't been set yet, but thorough preparation is needed before then, and that takes time," Kremlin spokesperson Dmitry Pesko.

The delay came after Russia reiterated to the U.S. its previous terms for reaching a peace deal, including that Ukraine cede control of the whole of the southeastern Donbas region.

That amounted to a rejection of Trump's statement last week that both sides should stop at the current front lines.

Russian Deputy Foreign Minister Sergei Ryabkov was quoted by state news agency RIA as saying he could not confirm that Moscow had conveyed its position as reported by Reuters.

"Preparations for the summit are continuing," Ryabkov was quoted as saying. "I don't see any major obstacles."

He added: "It's a difficult process, I admit – but that’s precisely what diplomats are for."

Russia and Ukraine pounded each other with heavy overnight missile attacks as renewed uncertainty surrounded the U.S.-led peace effort.

Ukrainian officials said on Wednesday that Russian attacks had killed six people, including two children, in Kyiv and the nearby region, and forced power outages nationwide.

Ukraine's military said late on Tuesday that it used Franco-British Storm Shadow air-launched missiles to strike a chemical plant in southern Russia's Bryansk region.