Russia's ground forces on Thursday crossed into Ukraine from several directions, Ukraine's border guard service said, hours after President Vladimir Putin announced the launch of a major offensive.

Russian tanks and other heavy equipment crossed the frontier in several northern regions, as well as from the Kremlin-annexed peninsula of Crimea in the south, the agency said.

It said one of its servicemen died in a shelling attack along the Crimean border, the first officially confirmed military death of the Russian invasion.

Ukraine has suffered heavy casualties in its eight-year conflict with Russian-backed rebels in the separatist east, but has reported no fatalities along its southern border with Crimea for some years.

After holding a series of emergency calls with world leaders, including US President Joe Biden, the Ukrainian leader convened a meeting of the top military brass, his office said.

"The armed forces of Ukraine are waging heavy combat," presidential adviser Mykhailo Podoliak said.

"We have losses," he added, without giving details.

"In several places, the Russian armed forces have been repelled."

Ukrainian officials said Russia was primarily targeting military infrastructure and silos, managing to push five kilometers deep along the norther frontier.

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