KYIV/MOSCOW. Russia on Tuesday faced increasing isolation over its invasion of Ukraine, with fierce resistance on the ground denying President Vladimir Putin decisive early gains despite heavy shelling and a huge military convoy outside the capital, Kyiv.

Ukrainian officials reported a Russian bombardment of Kharkiv, the country's second largest city, had killed dozens of civilians. It was not possible to independently verify the casualty figures.

"Barbaric rocket attacks and MLRS (multiple launch rocket systems) of peaceful cities are evidence that they are no longer able to fight armed Ukrainians," Ukrainian Defence Minister Oleksii Reznikov said on Facebook.

Ceasefire talks between Russia and its southern neighbour held Monday failed to reach a breakthrough and negotiators have not said when a new round would take place.

Putin faces mounting international pressure for last week launching the biggest assault on a European state since World War Two, and the systemic impact of Western sanctions led to a near 30% collapse in the rouble on Monday before central bank intervention rescued the currency from its lows.

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), US 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," US Republican Senator Marco Rubio said after a classified briefing with top Biden administration officials.

But fighting on several fronts was taking a toll.

Some 70 Ukrainian servicemen were killed on Sunday by Russian shelling of a base in the town of Okhtyrka, between Kharkiv and Kyiv, the regional governor said on Facebook.

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.

Turkey shuts straits to warships

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 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.

Public health experts say Ukraine is running low on critical medical supplies and fears of a wider public health crisis are growing as people flee their homes and health services and supplies are interrupted.

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

Private sector pullout

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.

A stream of companies pulling out of Russia is expected to grow on Tuesday.

Oil companies Shell (SHEL.L), BP and Norway's Equinor (EQNR.OL) have said they would exit positions in Russia, which relies on oil and gas for export earnings.

Canada said it would ban imports of Russian crude oil, and U.S. Republican Senator Lindsey Graham urged the Biden administration to target the Russian energy sector with sanctions.

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

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

Mastercard said it had blocked multiple financial institutions from its payment network as a result of sanctions on Russia and Visa (V.N) said it would take action too.

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 International Olympic Committee 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.