Russia's invasion, launched last week, appears not to have achieved the decisive early gains that Putin would have hoped for, while the impact of Western sanctions led to a near 30 per cent collapse in the rouble on Monday before central bank intervention.
Ceasefire talks held on 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 imposed sanctions on Russia's central bank, its top businesses, oligarchs and officials, including Putin himself, and barred some Russian banks from the SWIFT international payments system.
NATO ally Turkey delivered another blow to Moscow on Monday by warning warring countries not to send warships through its Bosphorus and Dardanelles Straits that separate the Black Sea from the Mediterranean, effectively bottling up Russia's Black Sea Fleet.
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.
But, the United States and its allies 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 64 km, US satellite company Maxar said.
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.Â
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.
Canada said it would ban imports of Russian crude oil, and US Republican Senator Lindsey Graham said the Biden administration should target the Russian energy sector with sanctions. Oil and gas are Russia's top export earners.
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 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 auto makers have cut shipments, ended partnerships and called Russia's actions unacceptable, with more considering similar actions.
Moves to isolate Russia have extended to culture and sports, as well.
Three major studios, Sony, Disney and Warner Bros., said they would pause theatrical releases of upcoming films in Russia while FIFA and the IOC moved to bar Russian teams and athletes from competing.
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.
