The United States closed its airspace to Russian planes as the Russian military attempted to encircle and subdue Ukrainian cities with intensifying bombardments on Wednesday, almost a week into an invasion that has sparked massive international sanctions.

Already shunned by the West over its invasion of Ukraine, Russia has shown no sign of stopping an assault that has included strikes on Kyiv and rocket attacks in the second city of Kharkiv. Dozens have been killed.

Russia has failed to capture a single city since its full-scale invasion began nearly a week ago, and Western analysts say Moscow has fallen back on tactics which call for devastating shelling of built-up areas before entering them.

Hundreds of thousands of Ukrainians have fled the fighting as a kilometers-long Russian military convoy north of Kyiv advances toward the city. West of Kyiv, in the city of Zhytomyr, four people, including a child, were killed on Tuesday by a Russian cruise missile, a Ukrainian official said.

Ukrainian President Volodymyr Zelenskiy called on Russia to stop bombarding civilians and resume talks. "It's necessary to at least stop bombing people, just stop the bombing and then sit down at the negotiating table," he told Reuters and CNN in a joint interview in a heavily guarded government compound in Kyiv.

Zelenskiy later thanked Western leaders for their support. "Today, more than ever, it is important for us to feel that we are not alone," he wrote on Twitter.

Russian President Vladimir Putin has drawn global condemnation and sanctions that have sent the rouble to historic lows and forced Russians to queue outside banks for their savings.

Putin ordered the "special military operation" last Thursday in a bid to disarm Ukraine, capture the "neo-Nazis" he says are running the country and crush its hopes of closer ties to the West.

In his first State of the Union address, US President Joe Biden said the United States would ban Russian planes from its airspace, joining the European Union and Canada, and vowed Putin would pay a heavy price for his decision to invade Ukraine.

"He has no idea what's coming," he said in the chamber of the House of Representatives. "Let each of us if you're able to stand, stand and send an unmistakable signal to Ukraine and to the world."

The lawmakers stood, applauded and roared, many of them waving Ukrainian flags.

'Freeze and seize'

Ukraine, which is not a member of Nato, has called on the US-led military alliance to implement a no-fly zone - a request rejected by Washington, which fears stoking a direct conflict between the world's two biggest nuclear powers.

Washington and its allies have instead sent weapons to Kyiv, and Treasury Secretary Janet Yellen said the United States had agreed with partners to convene a task force "to freeze and seize the assets of key Russian elites".

The move "will inflict financial pain on the powerful individuals surrounding Putin and make clear that no one is beyond our collective reach," Yellen said in a statement following a Tuesday call with Group of Seven officials.

The West is shutting off Russia's economy from the global financial system, pushing international companies to halt sales, cut ties, and dump tens of billions of dollars worth of investments.

Exxon Mobil joined other major Western energy companies including British BP PLC and Shell in announcing it would quit oil-rich Russia over the invasion.

Apple Inc stopped sales of iPhones and other products in Russia and was making changes to its Maps app to protect civilians in Ukraine. Alphabet Inc's Google dropped Russian state publishers from its news, and Ford Motor suspended operations in the country.

Russia on Tuesday placed temporary restrictions on foreigners seeking to exit Russian assets, meaning that billions of dollars worth of securities held by foreigners are at risk of being trapped.

Civilians killed

The heaviest Russian bombardment so far appeared to be around Ukraine's second-largest city Kharkiv, near the border with Russia. Dozens of residents including children were killed when a Russian strategic bomber fired 16 guided missiles toward a residential area on Monday, Ukraine's defence ministry said.

In Ukraine's largely Russian-speaking city of Donetsk, in territory controlled by Russian-backed separatists, authorities said three civilians had been killed by Ukrainian shelling.

Reuters was not able to confirm any of the incidents of reports of casualties. The United Nations says at least 136 civilians have been killed in the invasion, but that the real number of people is likely much higher.

Russia has not published any precise casualty figures for its own military but says its losses have been far lower than those of Ukrainian forces.

Ukraine's air force continued to defy expectations that Russia would achieve swift dominance of the air. "The airspace is actively contested every day," a senior US defense official said, speaking on condition of anonymity.