Ukrainian forces fought off Russian troops in the streets of the capital Kyiv on Friday as President Volodymyr Zelensky accused Moscow of targeting civilians and called for more international sanctions.

Pre-dawn blasts in Kyiv set off a second day of violence after Russian President Vladimir Putin defied Western warnings to unleash a full-scale invasion on Thursday that quickly claimed dozens of lives and displaced at least 100,000 people.

The United States and its allies responded with a barrage of sanctions, but the Russian forces looked to press home their advantage after a string of key strategic victories in their air and ground assault.

Zelensky recalled Nazi Germany's 1941 invasion and praised his people for "demonstrating heroism".

As Russian forces closed in on the city, Zelensky called on Europeans with "combat experience" to take arms and defend Ukraine, saying the West was too slow to help his country.  

Russian Foreign Minister Sergei Lavrov said Moscow wanted to "free Ukraine from oppression" and denied there had been any strikes on civilian targets.

He said Moscow was ready to talk if Ukraine's armed forces "lay down their arms", adding that "nobody intends to occupy Ukraine".

Trail of destruction

In Obolonsky, a northern district of Kyiv, what appeared to be an advance party of Russia's invasion force left a trail of destruction.

Pedestrians ran for safety and small arms fire and explosions were heard.

A dead man in civilian clothes lay sprawled on the sidewalk and, nearby, medics rushed to help another man whose car was crushed under the tracks of an armoured vehicle.

In contrast, the city centre felt like a ghost town.

Intersections around the government district were manned by green armoured vehicles and machine-gun toting soldiers in balaclavas.

Sirens wailed over the cloudy city at jarring intervals throughout the day. Booms of unexplained origin echoed across the deserted streets.

Russian forces first arrived on the outskirts of Kyiv on Thursday when helicopter-borne troops assaulted an airfield just outside the city, close to Obolonsky.

Ukraine said more forces were advancing from the north and northeast of the city.

As Russian forces arrived in Obolonsky, the Ukrainian defence ministry told civilians to resist.

"We urge citizens to inform us of troop movements, to make Molotov cocktails, and neutralise the enemy," it said.

Ukraine says 137 people, including soldiers and civilians, have been killed since Russia attacked.

After speaking to EU chief Ursula von der Leyen, Zelensky called for more sanctions, tweeting that "the pressure on Russia must increase".

'Better to die'

In the Ukrainian village of Starognativka near the frontline where separatists have faced off against Kyiv's forces for years, official Volodymyr Veselkin said on Friday missiles had been raining down all morning and the power was out.

"They are trying to wipe the village off the face of the earth," he said.

The UN's refugee agency said late Thursday that some 100,000 were already displaced inside Ukraine, while thousands of others fled across the border.

Streams of people in cars and on foot were seen crossing into Hungary, Poland and Romania and hundreds camped out in the Polish border city of Przemysl.

In Kyiv, many residents fled their homes and took shelter in the city's subway system.

Zelensky said there was now a "new iron curtain" between Russia and the rest of the world, adding later that his nation had been "left alone".

"Who is ready to fight alongside us? I don't see anyone."

And while the United States moved to impose sanctions on Russian elites and banks, it stressed that American forces would not fight in Ukraine.

NATO also said it would not send forces to Ukraine.

Among the highest-profile strategic developments on Thursday, Ukraine said Russian forces had seized the Chernobyl nuclear power plant -- prompting concern from international nuclear watchdogs.

Zelensky called the attack on Chernobyl "a declaration of war on all of Europe".

Russia said Thursday its forces had destroyed over 70 Ukrainian military targets, including 11 airfields.

Western intelligence confirmed Moscow had established "complete air superiority" over Ukraine.

Sanctions

Weeks of diplomacy failed to deter Putin, who massed over 150,000 troops on Ukraine's borders in what the West said was Europe's biggest military build-up since World War II.

Western allies had initially imposed some sanctions on Russia in an effort to stop Putin from invading, then followed through on Thursday with vows to heavily punish Russia economically.

US President Joe Biden announced export controls against Russia, alongside sanctions on Russian elites he called "corrupt billionaires", and banks.

He will meet Friday with fellow NATO leaders in an extraordinary virtual summit to discuss the security situation in and around Ukraine.

The EU moved to impose "massive" sanctions on Russia's energy and finance sectors and promised to adopt more.

A rare voice of support for Moscow came from the Myanmar junta which said Russia's invasion of Ukraine was "justified", while Syrian President Bashar al-Assad praised the invasion, saying it was a "correction of history".

Demonstrations across Europe

In a televised address, Putin justified the assault as a defence of the self-proclaimed Donetsk and Lugansk republics in eastern Ukraine.

The leaders of the two separatist territories asked Moscow for military help against Kyiv after Putin recognised their independence on Monday.

A conflict between the separatists and government forces has dragged on since 2014, killing more than 14,000 people.

Russia has also long demanded that Ukraine be forbidden from ever joining NATO and that US troops pull out from Eastern Europe.