"I know that more than 40 have been killed and several dozen wounded.

"I am aware of nearly 10 civilian losses," presidential administration aide Oleksiy Arestovych told reporters.

Earlier updates

Ukraine's military earlier stated that it had destroyed four Russian tanks on a road near the eastern city of Kharkiv, killed 50 troops near a town in Luhansk region and downed a sixth Russian aircraft, also in the country's east.

Russia has denied reports that its aircraft or armoured vehicles have been destroyed.

Ukraine's border guard service said that three of its servicemen had been killed in the southern Kherson region and that several more were wounded.

Russia's ground forces 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.

Timeline: Russia's stand-off with Ukraine

"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 kilometres deep along the norther frontier.

Russia says taking out military infrastructure at Ukraine's air bases

Russia's defence ministry said earlier it had taken out military infrastructure at Ukraine's air bases and "suppressed" its air defences, Russian news agencies reported.

The ministry denied reports that its aircraft had been downed over Ukraine. Earlier, Ukraine's military had said five Russian planes and one helicopter were shot down over its Luhansk region.

"The air defence assets of the Ukrainian armed forces have been suppressed," Interfax news agency quoted the ministry as saying.

"The military infrastructure of the Ukrainian Armed Forces' air bases has been taken out of action.

"Information in foreign media about a Russian plane allegedly being shot down is not true."

The Russian defence ministry had said it was targeting Ukrainian military infrastructure with precision weapons after Putin announced a military operation against the country.

"Military infrastructure, air defence facilities, military airfields, and aviation of the Armed Forces of Ukraine are being disabled with high-precision weapons," the defence ministry said in a statement carried by Russian news agencies.

The statement comes after Russian President Putin authorised a military operation in eastern Ukraine on Thursday in what appeared to be the start of war in Europe over Russia's demands for an end to NATO's eastward expansion.

Shortly after Putin spoke in a special televised address on Russian state TV, explosions could be heard in the pre-dawn quiet of the Ukrainian capital of Kyiv. Gunfire rattled near the capital's main airport, the Interfax news agency said.