Ukrainian authorities in besieged Mariupol said on Thursday about 15,000 civilians had been illegally deported to Russia since Russian forces seized parts of the southern port city.
Ukrainian officials say civilians trapped in Mariupol, which is normally home to about 400,000 people, face a desperate plight without access to food, water, power or heat.
Local authorities said on Sunday that thousands of residents had been taken by force across the border but did not provide a more precise figure. Russian news agencies said at the time that buses had carried several hundred people that Moscow calls "refugees" from Mariupol to Russia in recent days.
"Residents of the Left Bank district are beginning to be deported en masse to Russia. In total, about 15,000 Mariupol residents have been subjected to illegal deportation," the Mariupol City Council said in a statement issued on Thursday.
Russia denies targeting civilians in what President Vladimir Putin calls a "special military operation" to demilitarize and "denazify" Ukraine – which, along with the West say Putin launched an unprovoked war of aggression.
Deputy Prime Minister Iryna Vereshchuk told a video briefing that Ukrainian authorities were continuing efforts to secure an agreement from Russia to open a safe corridor to and from the southeastern coastal city.
Each side has blamed the other for the repeated failure to agree on arrangements to evacuate civilians from Mariupol, control of which would help Russia secure a land corridor to the Crimea peninsula that Moscow annexed from Ukraine in 2014.
Ukrainian President Volodymyr Zelensky said in a video address to Italy's parliament on Tuesday that there was "nothing left" in Mariupol after weeks of Russian bombardment.
A Reuters team that reached a Russian-controlled part of the destroyed city on Sunday described a wasteland of charred apartment blocks and bodies wrapped in blankets lying by a road.
Ukraine's leader called for solidarity on Thursday, a month since Russia's invasion began, warning he would see who sells out at summits in Europe where bolstering sanctions are being planned, but restrictions on energy could prove divisive.
The General Staff of the Armed Forces of Ukraine also reported the Russian total casualties as of Thursday, which includes 15,800 personnel, 530 tanks, 47 anti-aircraft warfare systems, 1,033 vehicles and four boats.
US President Joe Biden has arrived in Brussels for meetings of the alliance, G7 and European Union over the attack that began on February 24 and has caused more than 3.6 million refugees to flee the country.
Biden's visit could also shine light on a dispute with European allies, some of whom are heavily reliant on Russian oil and gas, over whether to impose further energy sanctions.
The issue has been a "substantial" topic and the subject of "intense back and forth" in recent days, White House national security adviser Jake Sullivan told reporters. The United States has already banned imports of Russian oil.
Putin on Wednesday said Moscow planned to switch gas sales made to "unfriendly" countries to roubles, causing European gas prices to soar on concerns the move would exacerbate the region's energy crunch.
Agreement has been reached on the establishment of seven humanitarian corridors to evacuate civilians from Ukrainian towns and cities on Thursday, Deputy Prime Minister Iryna Vereshchuk said.
Civilians trying to leave besieged Mariupol would find transport at nearby Berdyansk, she said, making clear Russia was not allowing a safe corridor to be created to or from the centre of the southern port city.
As the humanitarian toll from the conflict continues to rise, driving a quarter of Ukraine's population of 44 million from their homes, Zelensky called on people around the world to take to the streets and demand an end to the war.
"Come from your offices, your homes, your schools and universities, come in the name of peace, come with Ukrainian symbols to support Ukraine, to support freedom, to support life," he said in a video address.
The United States planned to announce more sanctions on Russian political figures and oligarchs on Thursday, and officials would have more to say on Friday about European energy issues, Sullivan said.
Ahead of his meeting with Biden, NATO Secretary-General Jens Stoltenberg said the alliance would boost its forces in Eastern Europe by deploying four new battle groups in Bulgaria, Hungary, Romania and Slovakia.
Zelensky said on Thursday he expected "serious steps" from Western allies.
He repeated his call for a no-fly zone and complained that the West had not provided Ukraine with planes, modern anti-missile systems, tanks or anti-ship weapons.
"At these three summits we will see who is our friend, who is our partner and who sold us out and betrayed us," he said in a video address released early on Thursday.
Moscow calls its actions in Ukraine a “special operation” that it says is not designed to occupy territory but to destroy its southern neighbor's military capabilities and capture what it regards as dangerous nationalists.
The West says this is a baseless pretext for an unprovoked war.
Although the Kremlin says its operation is going according to plan, Russian forces have taken heavy losses, stalled on most fronts and face supply problems. They have turned to siege tactics and bombardments, causing huge destruction and many civilian deaths.
Kyiv Mayor Vitali Klitschko told reporters on Wednesday that 264 civilians in the city had been killed by Russian attacks. He later said one person was killed and two wounded on Wednesday when shells hit a shopping center parking lot.
Russia has denied targeting civilians.
US Secretary of State Antony Blinken said the United States had assessed that members of Russia's forces had committed war crimes.
Blinken said there had been "numerous credible reports of indiscriminate attacks and attacks deliberately targeting civilians, as well as other atrocities."
The worst-hit has been the southern port of Mariupol, where hundreds of thousands of people have been sheltering since the war's early days under constant bombardment and with food, water and heating supplies cut.
Satellite photographs from commercial firm Maxar showed massive destruction of what was once a city of 400,000 people, with residential apartment buildings in flames.
A total of 4,554 people were evacuated from Ukrainian cities through humanitarian corridors on Wednesday, a senior official said, considerably fewer than the previous day.
Ukraine's armed forces chief of staff early on Thursday said Russia was still trying to resume offensive operations to capture the cities of Kyiv, Chernihiv, Sumy, Kharkiv and Mariupol.
To counter troop shortages, Moscow was moving in fresh units close to the Ukraine border and calling up soldiers who had recently served in Syria, it added in a Facebook post.
As Western leaders prepared to meet, British Prime Minister Boris Johnson said he would push for an increase in defensive lethal aid to Ukraine.
The first U.S. shipment from a new, $800 million arms package for Ukraine authorized last week will start flying out in the next day or so, a senior defense official said.
Putin's threat to switch certain gas sales to roubles sent European futures soaring on concerns the move would exacerbate an energy crunch and jam up deals that run to hundreds of millions of dollars every day.
Russian gas accounts for some 40% of Europe's total gas consumption.
Moscow has drawn up a list of "unfriendly" countries which have imposed sanctions. They include the United States, European Union members, Britain and Japan, among others.
"The changes will only affect the currency of payment, which will be changed to Russian roubles," said Putin.
And as an information battle also rages, a Russian regulator blocked Alphabet's news aggregator Google News, saying it allows access to what it calls fake material about the military operation, Interfax news agency said.
Google said in a statement that some people were "having difficulty" accessing the Google News app and website in Russia and this was "not due to any technical issues at our end."
Earlier the company said it would not help websites, apps and YouTube channels sell ads alongside content that it deemed exploits, dismisses or condons the conflict.
The Russian government has no interest in negotiating a ceasefire in Ukraine for now as its army has not reached its military goals, European Union's top diplomat Josep Borrell said on Thursday.
"Right now, Russia doesn't want to sit and negotiate anything: what it wants is to occupy the ground," Borrell said in an interview with Spanish TVE channel. "It wants to surround the coast to the border with Moldova and isolate Ukraine from the sea. It wants to negotiate in earnest only when it has secured a position of strength."
The European Union and its allies will keep on delivering military aid to the Ukrainian army, Borrell said.
"It is important because everything will be decided in the next 15 days," he said. "What will make history is the capacity of Ukrainians to resist."