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Russia launched co-ordinated missile and artillery attacks on Ukrainian cities on Saturday, including the capital, Kiev, as its invasion of Ukraine entered a third day.
Ukraine officials said Russian forces fired cruise missiles from the Black Sea at the cities of Sumy, Poltava and Mariupol, and there was heavy fighting near the southern city of Mariupol.
The Russian defence ministry said air and ship-based cruise missiles were used to attack military targets, and that its troops had captured the city of Melitopol in the south-east, Russia's Interfax news agency reported.
Emergency services in Kiev said a residential building was hit. They posted a picture online of the tower block with a hole covering at least five floors blasted into the side and rubble strewn across the street below.
The authorities said the number of victims was "being specified" and that an evacuation was under way.
Kiev Mayor Vitaly Klitschko said the building was hit by a Russian missile.
The bombardment follows an an overnight attack by Russian troops on an army base in central Kiev that was repulsed, Ukraine's military said.
"Military criminals of Russia attacked one of the military units in Kiev on Victory Avenue. The attack has been fought back," the armed forces of Ukraine said in an English-language post on Facebook.
Citing a threat to Russia’s security, President Vladimir Putin on Thursday sent Russian forces into Ukraine in a military operation that killed scores of people and forced more than 50,000 to flee Ukraine in just 48 hours.
A UN Security Council resolution calling on Russia to withdraw its troops was vetoed by Moscow on Friday.
Heavy, frequent artillery shelling and intense gunfire was heard away from Kiev’s city centre in the early hours of Saturday, witnesses said, hours after President Vlodymyr Zelenskyy said a Russian assault on Kiev was imminent.
"The fate of Ukraine is being decided right now," Mr Zelenskyy said in a video posted on his Telegram channel on Friday. "Tonight, they will launch an assault. All of us must understand what awaits us. We must withstand this night."
He released another video on Saturday morning to deny reports that he had asked the military to lay down arms.
"We will not lay down the weapons. We will defend our state because our weapon is our truth," he said.
Ukraine’s air force command reported heavy fighting near an air base at Vasylkiv, south-west of the capital, which it said was under attack from Russian paratroopers.
It also said one of its fighters had shot down a Russian transport plane.
Moscow said on Friday it had captured the Hostomel airfield north-west of the capital – a potential staging post for an assault on Kiev that has been fought over since Russian paratroopers landed there in the first hours of the war.
A picture of what was happening on the ground across Ukraine – the largest country in Europe after Russia – was slow to emerge.
Mr Zelenskyy wrote on Twitter that there had been heavy fighting with deaths at the entrance to the eastern cities of Chernihiv and Melitopol, as well as at Hostomel.
Witnesses said they heard explosions and gunfire near the airport in Kharkiv, Ukraine's second city, close to Russia's border. Ukraine's military said Russian troops had been stopped with heavy losses near the northeastern city of Konotop.
Britain's defence ministry said Russian armoured forces had opened a new route of advance towards the capital after failing to take Chernihiv.
Ukraine said more than 1,000 Russian soldiers had been killed. Russia did not release casualty figures. Mr Zelenskyy said late on Thursday that 137 soldiers and civilians had been killed in the fighting, with hundreds wounded.
An assessment of the military situation by the Institute for the Study of War (ISW), a Washington-based nonprofit, said Russia had fallen short of its objectives in the first two days of the invasion.
“Poorly planned and organised Russian military operations along Ukraine’s northern border have been less successful than those emanating from Crimea so far,” ISW said in a situation update late on Friday.
“Determined and well-organised Ukrainian resistance around Kiev and Kharkiv has also played an important role in preventing the Russian military from advancing with the speed and success for which it had reportedly planned.”
However, Russia will probably defeat Ukrainian regular military forces and secure their territorial objectives at some point in the coming days or weeks if Mr Putin continues the offensive, it said.
The invasion followed months of Russian military build-up along Ukraine's borders. Mr Putin refused to pull back his forces unless Nato scrapped plans to induct Ukraine as a member and reduced its presence in eastern Europe. The western military alliance rejected the demands.
Western allies have offered military assistance to Ukraine, while Nato has beefed up its forces in neighbouring member countries.
US President Joe Biden on Friday signed a memorandum directing the Department of Defence to provide up to $600 million in “immediate military assistance” to Ukraine.
Agencies contributed to this report