Explosions were heard in the capital Kyiv, 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.

Ukraine said it was hit by two waves of missile strikes - one shortly after Mr Putin's announcement and another mid-morning.

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

Ukraine's foreign minister Dmytro Kuleba described the assault as a "full-scale invasion" which was "from multiple directions".

"Peaceful Ukrainian cities are under strikes," he said. "This is a war of aggression. Ukraine will defend itself and will win. The world can and must stop Putin. The time to act is now."

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

What's happening on the ground

Ukraine's leadership said at least 40 Ukrainian soldiers have been killed and at least several dozen wounded in what it called a "full-scale war" targeting the country from the east, north and south.

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

An adviser to Ukraine's interior minister said Russian troops from the direction of Russian ally Belarus entered an area near the Chernobyl former nuclear power plant - the site of a devastating 1986 accident - with reports heavy fighting may disturb nuclear waste there.

Ukraine's president Volodymyr Zelenskyy said on Twitter that Russian forces were trying to seize the Prypyat 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."

Prypyat is around 100 miles from Kyiv.

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 the local Ukrainian leadership.

At least 203 attacks have been carried out throughout the entire country today, according to Ukrainian police.

Armed forces are fighting a massive Russian artillery bombardment of infrastructure facilities, an adviser in President Zelenskyy's office said.

Ukraine's military said it destroyed four Russian tanks on a road near the eastern city of 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.

Heavy fighting is going on in eastern Ukraine and around the Sumy, Kharkiv, Kherson and Odessa regions, an adviser to Ukraine's presidential office said.

Explosions heard in Kyiv as nearby airport targeted

The Russian military is trying to break into the Kyiv region, according to Ukraine's border guard.

Explosions were witnessed by Sky News both in and around Kyiv and air raid sirens went off in the city.

Russian helicopters attacked Gostomel, a military airport near Kyiv, and Ukraine shot down three of them, according to Ukrainian officials.

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.

Kharkiv, Lviv and Mariupol targeted

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

In Kharkiv, a city in the east, a boy was killed after an apartment building was shelled.

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

Ukrainian tanks were seen heading to the city by Reuters news agency and an airport there caught fire.

Five Russian planes and one helicopter were shot down over the Luhansk region, according to the Ukrainian military.

Martial law introduced as Ukrainian border attacked

Ukrainian President Zelenskyy introduced martial law, urging people to stay home as much as possible and not panic.

The Russian military attacked the border from neighbouring Belarus as well as annexed Crimea, according to Ukraine's border agency.

Border units, border patrols and checkpoints were targeted with artillery, heavy equipment and small arms, it said. Missiles were also used.

Ukraine said it is facing non-stop cyber attacks.

Read more:
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How does Ukraine's military compare to Russia's?

People have started fleeing some cities and the country has closed its airspace to civilian flights.

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."