Russian President Vladimir Putin said Western sanctions were akin to war as his forces pressed their assault on Ukraine on Saturday (local time) for a 10th day.
Moscow and Kyiv traded blame over failed plans for a brief ceasefire to enable civilians to evacuate two cities besieged by Russian forces. Russia's invasion has already driven nearly 1.5 million refugees westward into the European Union.
Ukraine's President Volodymyr Zelensky made a "desperate plea" for eastern Europe to provide Russian-made aircraft to his country during a video call with US senators on Saturday, the chamber's majority leader, Chuck Schumer, said.
NATO, which Ukraine wants to join, has resisted Zelensky's appeals to impose a no-fly zone over his country, saying this would escalate the conflict outside Ukraine.
But there is strong bipartisan support in the US Congress for providing US$10 billion (A$13.5 billion) in emergency military and humanitarian aid to Ukraine.
Putin said he wanted a neutral Ukraine that had been "demilitarised" and "denazified", adding: "These sanctions that are being imposed are akin to a declaration of war but thank God it has not come to that."
Ukraine and Western countries have rejected Putin's arguments as a baseless pretext for invading and have sought to squeeze Russia hard with swift and severe economic sanctions on its banks, oligarchs and others.
Israeli Prime Minister Naftali Bennett met with Putin at the Kremlin on Saturday to discuss the crisis before then speaking to Zelensky, Bennett's spokesperson said.
Israel has offered to mediate in the conflict, though officials have downplayed expectations for a breakthrough.
Ukrainian negotiators said a third round of talks with Russia on a ceasefire would go ahead on Monday, although Moscow was less definitive.
Two previous rounds were unsuccessful and Zelensky has said Russia must first stop bombing.
"Together we will all rebuild our state," Zelensky told Ukrainians in a televised address on Saturday evening.
"My confidence in this is reinforced by the energy of our resistance, our protest."
No evacuations after Russia accused of not observing ceasefire
Earlier, the International Committee of the Red Cross had said planned civilian evacuations from Mariupol and Volnovakha were unlikely to start on Saturday.
The city council in Mariupol had accused Russia of not observing a ceasefire, while Moscow said Ukrainian "nationalists" were preventing civilians from leaving.
Britain said the proposed ceasefire in Mariupol – which has been without power, water and heating for days – was likely an attempt by Russia to deflect international condemnation while it resets its forces.
The port of Mariupol has endured heavy bombardment, a sign of its strategic value to Moscow due to its position between Russian-backed separatist-held eastern Ukraine and the Black Sea Crimean peninsula, which Moscow seized from Kyiv in 2014.
Russia's Defence Ministry said its forces were carrying out a wide-ranging offensive in Ukraine and had taken several towns and villages, Interfax news agency reported.
In aerial combat near Zhytomyr, about 100 kilometres west of Kyiv, it said, four Ukrainian Su-27 fighter jets had been shot down. Reuters could not independently confirm the report.
A United Nations monitoring mission said at least 351 civilians had been confirmed killed and 707 injured in Ukraine since the start of the invasion on February 24, adding the real figures were likely to be "considerably higher".
The number of refugees could rise to 1.5 million by Sunday night from 1.3 million now, the UN refugee agency chief said.
Women and children, often numb with exhaustion, continued to pour into Poland and other neighbouring countries as well as into western Ukrainian cities such as Lviv.
"I've barely slept for 10 days," Anna Filatova, arriving in Lviv with her two daughters from heavily bombed Kharkiv, Ukraine's second city near its eastern border with Russia, said.
"The Russians want to flatten Kharkiv... We hate Putin."
Do you have a story tip? Email: firstname.lastname@example.org.