Moscow faced increasing isolation on Tuesday as President Vladimir Putin showed no sign of stopping an invasion of Ukraine, where fierce fighting and Russian bombardment have killed dozens and sparked a refugee crisis.

Russia's invasion, launched last week, appears not to have achieved the decisive early gains that Putin would have hoped for. Ceasefire talks held Monday failed to reach a breakthrough and negotiators have not said when a new round would take place.

The United States and its allies have sought to punish Russia economically for staging the biggest assault on a European state since World War Two. They have imposed sanctions on Russia's top businesses, oligarchs and officials, including Putin himself.

But Washington has ruled out sending troops to fight Russia or enforcing a no-fly zone as requested by Ukraine, fearing an escalation between the world's top two nuclear powers.

The United States and its allies have instead promised military aid to Kyiv, as President Volodymyr Zelenskiy warned the capital was under constant threat.

"For the enemy, Kyiv is the key target," Zelenskiy said in a video message late on Monday. "We did not let them break the defence of the capital, and they send saboteurs to us ... We will neutralise them all."

Zelenskiy said Russia, which calls its actions in Ukraine a "special operation", was targeting a thermal power plant providing electricity to Kyiv, a city of 3 million people.

Human rights groups and Ukraine's ambassador to the United States accused Russia of using cluster bombs and vacuum bombs. The United States said it had no confirmation of their use.

Staging a push for the capital, Russia has massed a convoy of armoured vehicles, tanks and other military equipment that stretches about 40 miles (64 km), U.S. satellite company Maxar said.

"What I think is pretty certain is Russia is off their timeline. I think they thought that within 72 hours they'd hold Kyiv," U.S. Republican Senator Marco Rubio said after a classified briefing with top Biden administration officials.

Russia says its actions are not designed to occupy territory but to destroy its southern neighbour's military capabilities and capture what it regards as dangerous nationalists.

Fighting has raged around the port of Mariupol and in the eastern city of Kharkiv, where Ukrainian officials said Russian artillery attacks had killed dozens of civilians, including children. It was not possible to verify those figures.

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More than 500,000 people have fled Ukraine, according to the United Nations refugee agency, setting off a refugee crisis as thousands await passage at European border crossings.

At least 102 civilians in Ukraine have been killed since the invasion started Thursday, but the real figure could be much higher, the U.N.'s human rights chief said.

Canada said it would ban imports of Russian crude oil, and U.S. Republican Senator Lindsey Graham said the Biden administration should target the Russian energy sector with sanctions. Oil is Russia's top export earner.

"We're not using the energy sector as a weapon," Graham told reporters. "We're failing to hit Putin where it hurts the most."

A stream of companies pulling out of Russia is expected to grow on Tuesday and deal further blows to the country's economy.

Shell, BP and Norway's Equinor all said they would exit positions in Russia, putting pressure on other Western companies with stakes in Russian oil and gas projects, such as ExxonMobil and TotalEnergies.

Leading banks, airlines, and automakers have cut shipments, ended partnerships and called Russia's actions unacceptable, with more considering similar actions.

The United States imposed new sanctions on Russia's central bank and other sources of wealth on Monday, triggering a plunge in the value of the rouble, and over the weekend some Russian banks were barred from the SWIFT international payments system.

Moves to isolate Russia have extended to culture and sports, as well.

Two major Hollywood studios, Disney and Warner Bros. said they would pause theatrical releases of upcoming films in Russia in response to its invasion of Ukraine and the unfolding humanitarian crisis.

Putin, who takes pride in athleticism and is passionate about martial arts, had his honorary black belt from World Taekwondo stripped from him over the invasion, the group said.