MOSCOW: Russian forces captured the southeastern Ukrainian city of Melitopol on Saturday, Russia's Interfax news agency reported, as Moscow launched coordinated cruise missile and artillery strikes on several cities, including the capital Kyiv.

Ukrainian officials were not immediately available for comment on the fate of Melitopol, a city of about 150,000 people. If the Interfax report citing the Russian defence ministry is confirmed, it would be the first significant population centre the Russians have seized since their invasion began on Thursday.

Earlier, Ukrainian officials said Russian forces fired cruise missiles from the Black Sea at Mariupol, also in the southeast, as well as Sumy in the northeast and Poltava in the east.

Kyiv authorities said a missile hit a residential building, and a Reuters witness said another hit an area near the airport. There was no immediate word on casualties. Gunfire erupted near city-centre government buildings at around dawn, a Reuters witness said. The cause was not clear.

President Volodymyr Zelenskiy, speaking in a video message from outside his Kyiv office, was defiant.

"We will not put down weapons, we will defend our state," he said.

After weeks of warnings from Western leaders, Russian President Vladimir Putin invaded from the north, east and south, an attack that threatens to upend Europe's post-Cold War order.

Putin said he had to eliminate what he called a serious threat to his country from its smaller neighbour and he cited the need to "denazify" Ukraine's leadership, accusing it of genocide against Russian-speakers in eastern Ukraine.

Kyiv and its Western allies dismiss the accusations as baseless propaganda.

In a televised meeting with Russia's Security Council on Friday, Putin appealed to Ukraine's military to overthrow their "neo-Nazi" leaders.

"Take power into your own hands," he said.

'UNDER CONTROL'

Western countries have announced a barrage of sanctions on Russia, including blacklisting its banks and banning technology exports. But they have stopped short of forcing it out of the SWIFT system for international bank payments.

At the United Nations, Russia vetoed a draft Security Council resolution deploring its invasion, while China abstained, which Western countries took as proof of Russia's isolation. The United Arab Emirates and India also abstained while the remaining 11 members voted in favour.

The White House asked Congress for $6.4 billion in security and humanitarian aid for the crisis, officials said, and Biden instructed the U.S. State Department to release $350 million in military aid. Read full story

Russia's defence ministry said their forces used air- and ship-based cruise missiles to carry out overnight strikes on military targets in Ukraine, Interfax said.

It said Russian troops had hit hundreds of military infrastructure targets and destroyed several aircraft and dozens of tanks and armoured and artillery vehicles.

Ukraine's air force command earlier said one of its fighters had shot down a Russian transport plane. Reuters could not independently verify the claim.

Mykhailo Podolyak, adviser to the president's office, said the situation in Kyiv and its outskirts was under control.

"There are cases of sabotage and reconnaissance groups working in the city, police and self-defence forces are working efficiently against them," Podolyak said.

Ukrainian authorities have urged citizens to help defend Kyiv from the advancing Russians. Some families took cover in shelters and hundreds of thousands have left their homes to find safety, according to a U.N. aid official.

Ukraine said more than 1,000 Russian soldiers had been killed. Russia did not release casualty figures. Zelenskiy said late on Thursday that 137 soldiers and civilians been killed with hundreds wounded.

Ukrainians voted overwhelmingly for independence at the fall of the Soviet Union and Kyiv hopes to join NATO and the EU - aspirations that infuriate Moscow.

Putin says Ukraine, a democratic nation of 44 million people, is an illegitimate state carved out of Russia, a view Ukrainians see as aimed at erasing their more than thousand-year history.

'READY TO TALK'

The United States imposed sanctions on Putin, Foreign Minister Sergei Lavrov, Defence Minister Sergei Shoigu and Chief of General Staff Valery Gerasimov. The European Union and Britain earlier froze any assets Putin and Lavrov held in their territory. Canada took similar steps.

The invasion triggered a flurry of credit rating moves on Friday, with S&P lowering Russia's rating to "junk" status, Moody's putting it on review for a downgrade to junk, and S&P and Fitch cutting Ukraine on default worries. 

But even as the fighting grew more intense, the Russian and Ukrainian governments signalled an openness to negotiations, offering the first glimmer of hope for diplomacy since Putin launched the invasion.

A spokesman for Zelenskiy said Ukraine and Russia would consult in coming hours on a time and place for talks.

The Kremlin said earlier it offered to meet in the Belarusian capital Minsk after Ukraine expressed a willingness to discuss declaring itself a neutral country, while Ukraine had proposed Warsaw as the venue. That, according to Russian spokesman Dmitry Peskov, resulted in a "pause" in contacts.

"Ukraine was and remains ready to talk about a ceasefire and peace," Zelenskiy spokesman Sergii Nykyforov said in a Facebook post. "We agreed to the proposal of the President of the Russian Federation."

But U.S. State Department spokesman Ned Price said Russia's offer was an attempt to conduct diplomacy "at the barrel of a gun" and Putin's military must stop bombing Ukraine if it was serious about negotiations.

READ MORE: Russia-Ukraine crisis: What led to the attacks and the latest developments