The number of Ukrainians fleeing the Russian invasion was expected to reach 1.5 million on Sunday as Ukraine pressed the West to toughen sanctions and deliver more weapons to repel Moscow's attack now in its 11th day.

Ukrainian police said there was relentless Russian shelling and air raids in the northeast Kharkiv region, reporting many casualties, while the U.N. World Health Organization said there had been several attacks on Ukrainian health care facilities.

Moscow and Kyiv have traded blame over Saturday's failed cease-fire to allow civilians to flee Mariupol and Volnovakha, two southern cities besieged by Russian forces. Ukraine said more talks were set for Monday, but Russia was less definitive.

People who have been able to escape Ukraine spilled into neighbouring Poland, Romania, Slovakia and elsewhere. Ukrainian President Volodymyr Zelenskyy has called on those in areas occupied by Russian troops to fight.

"We must go outside and drive this evil out of our cities," he said in an address on Saturday night.

British military intelligence said on Sunday that Russian forces were targeting populated areas in Ukraine, comparing the tactics to those Russia used in Chechnya in 1999 and Syria in 2016. But it said Ukrainian resistance was slowing the advance.

"The scale and strength of Ukrainian resistance continue to surprise Russia," British military intelligence said.

Russia has repeatedly denied targeting civilian areas.

"Attacks on healthcare facilities or workers breach medical neutrality and are violations of international humanitarian law," WHO chief Tedros Adhanom Ghebreyesus wrote on Twitter, saying the organization had confirmed attacks on "several" health care centers, causing multiple deaths and injuries.

He made no mention of Russia in his tweet.

Russian President Vladimir Putin, who launched what he calls a "special military operation" on Feb. 24, reiterated that he wanted a neutral Ukraine that had been "demilitarised" and "denazified." He likened Western sanctions "to a declaration of war," adding: "Thank God it has not come to that."

Ukraine and Western countries have decried Putin's reasons as a baseless pretext for the invasion and have imposed sweeping sanctions aimed at isolating Moscow and crippling its economy.

Sanctions and weapons

Ukraine's Foreign Minister Dmytro Kuleba, after meeting U.S. Secretary of State Antony Blinken at the Ukraine-Poland border, said he expected new sanctions and weapons for Ukraine in coming days.

The United States has promised to send more weapons and has said it could escalate sanctions. President Joe Biden has sought $10 billion in emergency funding to respond to the crisis.

Washington is working with Poland as Warsaw considers whether to provide fighter jets to Ukraine, the White House said.

Zelenskyy asked for help securing aircraft from European allies in a video call with U.S. lawmakers. He again called for more lethal aid, a ban on Russian oil, a no-fly zone and an end to Visa Inc. and MasterCard Inc. privileges in Russia, U.S. media reported. Visa and Mastercard later said they would suspend credit card operations in Russia, the latest in a dramatic series of corporate pullbacks over the invasion.

NATO, which Ukraine wants to join, has resisted Zelenskyy's appeals to impose a no-fly zone over his country, saying it would escalate the conflict outside Ukraine.

Seeking to mediate, Israeli Prime Minister Naftali Bennett met Putin at the Kremlin on Saturday and later spoke to Zelenskyy.

"We continue dialogue," Zelenskyy tweeted after the call.

Russia told the EU and NATO to stop the "pumping of state-of-the-art weapons systems" into Kyiv, Russian Foreign Ministry spokesperson Maria Zakharova said, according to RIA.

Russian media cited an unidentified source on Sunday as saying Ukraine was close to building a plutonium-based "dirty bomb" nuclear weapon, although the source cited no evidence.

'Severe impact'

Shortly before the invasion, Putin had said Ukraine was using Soviet know-how to create its own nuclear weapons, and that this was tantamount to preparation for an attack on Russia.

Ukraine's government has said it had no plans to rejoin the nuclear club, after giving up its nuclear arms in 1994 following the break-up of the Soviet Union.

The International Monetary Fund warned the conflict would have a "severe impact" on the global economy, driving up energy and grain prices. It said it would weigh Kyiv's request for $1.4 billion in emergency financing as early as this week.

Ukraine's military said more than 11,000 Russian troops had been killed so far and 88 Russian aircraft shot down. Reuters could not corroborate the claim.

In Mariupol, encircled by Russian forces and shelled for days, the situation was grim, with no power or water.

"We can’t collect all the bodies on the street," Deputy Mayor Sergei Orlov told CNN, saying it was impossible to count civilian deaths there.

Medical charity Doctors Without Borders (MSF) said it was rushing more emergency supplies to Ukraine as hospitals faced shortages.

More than 350 civilians have been killed, according to the U.N. rights office, with hundreds more injured.

Heavy shelling was heard in the background as residents of Volnovakha tried to flee.

"Help us if you can, we all want to live, we have kids, husbands, we are mothers and fathers, we are also people," said one local, Larisa. "Where shall I go?"

The mayor of the western city of Lviv said more than 65,000 refugees passed through its train station on Friday alone.

The United Nations High Commissioner for Refugees estimated the number of refugees could swell to 4 million by July.