Russian President Vladimir Putin told his citizens he had begun a "specialised military operation".

Where is military action taking place?

Shortly after Mr Putin made his announcement on state television, just before 3am GMT, large explosions were heard in Kyiv, Kharkiv, Odessa and other cities across Ukraine.

In the capital, Kyiv, gunfire could be heard near the airport, the Russian Interfax news agency said.

Ukrainian media reported that military command centres there had been struck by missiles.

Sky News's Stuart Ramsay, who is in Kyiv, said he was woken up by loud explosions.

"There are major attacks happening," he said. Air raid sirens were heard going off in the city.

• Ivano-Frankvisk - reports of an explosion sited near an airbase
• Podilsk - reports of at least six Ukrainian soldiers killed
• Kyiv - explosions heard and a fire reportedly broke out near the river
• Ochakiv - fire reportedly seen at the port
• Mariupol - Ukrainian tank column seen heading for the city
• Kharkiv and Chernihiv - deep breakthrough of Russian troops reported near those cities
• Lviv - shelling reported nearby
• Senkivka border post - images appear to show Russian troops crossing the border

In Kharkiv, in the northeast of the country, command centres were also hit by missiles, Ukrainian media reported.

Explosions could be heard, a witness told the Reuters news agency, and the windows in his apartment block had been shaking.

Ukraine's military said it had destroyed four Russian tanks on a road near Kharkiv.

In addition, it said it had killed 50 troops near a town in Luhansk region and downed a sixth Russian aircraft, also in the east of the country.

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

Ukraine said Russia was shelling across country all the way to the western Lviv region, 70 km from the border with Poland.

Russia is also moving military equipment into Ukraine from annexed Crimea, Ukrainian authorities said.

And in the southern port cities of Odessa and Mariupol, Russian troops have reportedly landed.

A witness heard three loud blasts in Mariupol, Reuters said.

According to Interfax, Russian-backed separatists said they had launched an offensive on the Ukrainian-controlled town of Shchastia in the east.

Explosions are also said to have rocked the breakaway eastern Ukrainian city of Donetsk.