KYIV/MOSCOW, March 1 (Reuters) - 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, has failed to achieve 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, fearing an escalation between the world's top two nuclear powers. It and its allies have instead promised military aid to Kyiv, as President Volodymyr Zelenskiy warns the capital is 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 that Russia, which calls its actions in Ukraine a "special operation", was targeting a thermal power plant that provides electricity to Kyiv, a city of 3 million people.
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.
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), the U.S. satellite company Maxar said.
Washington offered more support for Ukraine in the form of sanctions and weapons, Ukrainian Foreign minister Dmytro Kuleba said on Tuesday following a call with U.S. Secretary of State Antony Blinken.
"Ukraine craves for peace, but as long as we are under Russia's assault we need more sanctions and weapons. Secretary assured me of both," Kuleba wrote on Twitter.
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More than half a million people have fled Ukraine to neighbouring countries, according to the United Nations refugee agency, setting off a refugee crisis as thousands await passage at European border crossings.