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

Ukrainian police have said Russia has carried out 203 attacks since Thursday morning and fighting is going on across almost every part of the country.

Where is military action taking place?

• Ivano-Frankvisk - missile understood to have hit Ukrainian air base
• 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 - deep breakthrough of Russian troops reported along with death of young boy
• Lviv - shelling reported nearby
• Senkivka border post - images appear to show Russian troops crossing the border
• Luhansk - Ukraine claims it has killed 50 Russian troops
• Chuhuiv - Sky News understands missile has struck air base
• Odessa - Ukraine says at least 18 people killed in missile strike
• Sumy and Kherson - Ukraine claims fierce fighting underway
• Donetsk - explosions reported

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

Kharkiv

In Kharkiv, in the northeast of the country, command centres have been 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.

Shelling has also been reported across the city, with Ukraine's emergency service reporting a young boy had been killed after an apartment building came under fire.

An adviser to Ukraine's presidential office added that more than 40 Ukrainian soldiers were dead and dozens have been wounded.

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

Luhansk

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

Russia has denied reports that its aircraft or armoured vehicles were destroyed and has claimed to have downed two Ukrainian planes.

Kyiv

In Kyiv, the Ukrainian capital, an explosion has been heard, with Sky's Mark Austin reporting that smoke could be seen rising over buildings.

Military command centres in several cities, including Kyiv, have been targeted by missile strikes, according to a Ukrainian interior ministry official.

Following the explosion, an adviser to Ukraine's President Volodymyr Zelenskyy said the country was being hit by a "second wave" of missile strikes.

Read more: Ukraine in pictures - Explosions damage cities as people take shelter in subway stations

Ukrainian border officials have also claimed Russian helicopters have attacked Gostomel, a military airport near the capital.

Shortly after, Ukraine's internal ministry said it had downed three Russian helicopters - although it is not clear whether these are the same aircraft.

A Ukrainian Armed Forces' Antonov aircraft has also been shot down in Kyiv, according to the state emergency service.

Ivano-Frankivsk and Chuhuiv

In Ivano-Frankivsk, Sky News believes, after reviewing a video through an extensive verification process, that a missile strike has taken place on a Ukrainian air base.

Sky News also understands, after reviewing, verifying and corroborating a video, that a missile has hit another Ukrainian air base in Chuhuiv.

Lviv and Crimea

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

Ukraine's border guard agency said the Russian military attacked from neighbouring Belarus and that Russia has been moving military equipment from annexed Crimea into Ukraine.

Odessa, Sumy and Kherson

In the southern port city of Odessa, Russian troops have reportedly landed, with an adviser to Ukraine's presidential office saying fierce fighting is taking place.

Regional authorities said at least 18 people had been killed in a missile attack in the city.

A presidential adviser added that fighting was underway in Sumy, Kherson and Kharkiv as well.

Mariupol

Russian soldiers have also reportedly arrived in Mariupol, where an AP photographer said explosions could be heard.

Pictures showed damaged radar arrays and other equipment at a Ukrainian military facility just outside the southeastern city.

Shchastia, Chernihiv and Donetsk

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

In the Chernihiv region of northern Ukraine, an adviser to the Ukrainian president has said Russian forces are regrouping to continue their offensive.

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

How did the invasion begin and what is Putin's aim?

In the early hours of Thursday morning, Russia's President Vladimir Putin delivered a state television address where he said he had ordered a "specialised military operation" in Ukraine's eastern Donbas region.

He claimed that Russia does not intend to occupy Ukraine but instead will move to "demilitarise" it.

Russia was acting in self-defence and that the use of Ukraine's territory by NATO was unacceptable.

Read more: Inside the mind of Putin - Why is he invading Ukraine and what does he want?

Shortly after his announcement, large explosions were heard in Kyiv, Kharkiv, Odessa and other cities across Ukraine.

Sky News's Stuart Ramsay, who is in Kyiv, said he was woken up by loud explosions and air raid sirens were heard going off in the city.

Ukraine's President Volodymyr Zelenskyy then introduced martial law and urged people to stay at home.