Ukraine reported troops pouring across the borders with Russia and Belarus from the north and east, and landing on the coasts from the Black Sea in the southwest and Azov Sea in the southeast.

Fighting was reported near the former Chernobyl nuclear power plant, around 100 miles from the capital Kyiv, with Russian forces said to be trying to seize the site where there was a devastating accident in 1986.

And Ukraine's border guard said the Russian military was trying to break into the Kyiv region, with air raid sirens going off in the capital.

Missiles rained down on Ukrainian cities after President Vladimir Putin ordered a "specialised military operation" in the early hours.

Ukraine 'attacked from Russia, Belarus and Crimea' - latest live updates

Key developments at a glance:
• '40 dead' as Russia launches 'second wave'
• Ukrainian president reacts: Russian attack 'like fascist Germany in the Second World War'
• PM promises "massive sanctions" to "hobble" Moscow's economy
• UEFA to announce that the Champions League final to be moved from Russia

Explosions were heard in Kyiv, along with Kharkiv in the east, and Odessa in the south, and Sky News understands that borders have been breached in at least four areas of Ukraine.

Russia said its military had destroyed 83 Ukrainian land-based targets and it had achieved all its goals for Thursday.

A US official said Russian forces were moving on three fronts - from annexed Crimea to Kherson in the south, from Belarus - north of Ukraine - towards Kyiv, and from the northeast to Kharkiv.

They said in the first few hours of the assault, more than 100 missiles were launched from land and sea, primarily short-range ballistic missiles, but also medium-range ballistic missiles, cruise missiles, and surface-to-air missiles.

The main targets were barracks, ammunition warehouses, and 10 airfields, before ground forces moved in.

What's happening on the ground

Ukraine reported columns of troops penetrated its borders into the eastern Chernihiv, Kharkiv and Luhansk regions, and landed by sea at the cities of Odessa and Mariupol in the south.

Fierce fighting was taking place in Sumy and Kharkiv in the northeast, Kherson and Odessa in the south, and at a military airport near Kyiv, an adviser to the Ukrainian presidential office said.

Border units, border patrols and checkpoints were targeted with artillery, heavy equipment and small arms fire, Ukraine's border agency said.

'Difficult situation in Kharkiv'

Ukraine's president Volodymyr Zelenskyy said a difficult situation was developing in the country's second-largest city Kharkiv, about 12 miles from the Russian border, while in the north the Russians were slowly advancing towards Chernihiv.

In Kharkiv, a boy was killed after an apartment building was shelled.

Mr Zelenskyy also said a Russian airborne force in Hostomel airport outside Kyiv had been stopped.

The president said many Russian warplanes and armoured vehicles were destroyed but did not give numbers. He also said an unspecified number of Russian troops were captured.

Ukrainian officials earlier said three Russian helicopters had been shot down.

Russian forces 'trying to seize Chernobyl'

Mr Zelenskyy added on Twitter that Russian forces were trying to seize the Chernobyl plant and his troops were "giving their lives so that the tragedy of 1986 will not be repeated". He added: "This is a declaration of war against the whole of Europe."

And in the breakaway Donetsk region, where Ukraine government forces have been battling Russian-backed separatists for years, Russian forces struck a hospital, killing four people, according to local Ukrainian leaders.

'At least 40 Ukrainian soldiers killed'

Ukraine's leadership earlier said at least 40 Ukrainian soldiers were killed and at least several dozen wounded in what it called a "full-scale war" targeting the country from three sides.

Ukraine 'kills 50 Russian soldiers'

Ukraine's military said it destroyed four Russian tanks on a road near Kharkiv, killed 50 soldiers near a town in the Luhansk region and brought down six Russian warplanes in the east.

Russia's defence ministry said its country's military had destroyed 74 Ukrainian military facilities, including 11 air bases.

The Russian ministry said it was not targeting cities, but using precision weapons, claiming that "there is no threat to the civilian population".

A Ukrainian military plane was shot down and five people were killed, Ukrainian authorities said.

Ukraine's state emergency service said its ammunition depot in Kyiv caught fire. At least six people were killed in the nearby city of Brovary, according to the mayor.

At least 18 people were killed in a missile attack in the southern region of Odessa, according to regional authorities.

Shelling in the Lviv region has been reported by the Ukraine emergency service, while blasts were reported in Mariupol, a southern port city.

Martial law has been introduced in Ukraine with people being urged to stay home as much as possible and not panic. The country said it is facing non-stop cyber attacks and has closed its airspace to civilian flights.

In Kyiv, queues of people waited to withdraw money and buy supplies of food and water. Cars stretched for dozens of miles on the highway leading west towards Poland, where Western countries have prepared to receive hundreds of thousands of refugees.

Read more:
What is Putin thinking?
What is happening in Ukraine?
How does Ukraine's military compare to Russia's?

Putin's justification

In a television address announcing the "specialised military operation" in the Donbas region, Mr Putin claimed the attack was needed to protect people subjected to "genocide".

He accused the US and its allies of ignoring Russian demands to prevent Ukraine from joining NATO.

He claimed that Russia does not intend to occupy Ukraine but was aiming for the country's "demilitarisation" and "de-Nazification".

"Whoever would try to stop us and further create threats to our country, to our people, should know that Russia's response will be immediate and lead you to such consequences that you have never faced in your history," he said.

"We are ready for any outcome."