As Vladimir Putin turned to more brutal military tactics on the sixth day of the invasion, Russian troops stormed Ukrainian cities and mobilized tens of thousands of troops to surround Kyiv.

A missile hit the center of Kharkiv in one of the most destructive attacks on the densely populated urban area since Moscow’s military offensive, with Ukraine’s President Volodymyr Zelensky accusing Russia of guilty of war crimes.

An estimated 660,000 civilians have fled Ukraine for neighbouring countries since the conflict began, with the UN warning the movement could become “Europe’s biggest refugee crisis this century”.

In a sign of Russia’s growing international isolation, China said it “regrets the outbreak of the conflict” and is “extremely concerned about the harm to civilians”. “The situation in Ukraine has undergone tremendous changes,” Chinese Foreign Minister Wang Yi said in a phone call with his Ukrainian counterpart.

Russia’s offensive in Ukraine has sent diplomatic, economic and humanitarian shock waves, upended the postwar order in Europe and triggered the most far-reaching sanctions regime ever imposed on the world’s major economies.

EU member states will discuss on Tuesday cutting off seven Russian banks, including its second-largest lender VTB, from the Swift messaging network. It marks the latest in a series of rolling measures that have effectively cut off Moscow from key parts of the world financial system, as well as from international sporting and cultural events.

Thanks to five days of unexpectedly slow progress, Russia has changed its approach on the battlefield, pouring more troops and armor into Ukraine and stepping up its use in cities such as Kharkiv, Chernihiv and Kherson Artillery and other unguided weapons.

Ukraine’s second-largest city, Kharkiv, was hit by rockets on Tuesday, targeting a local government building engulfed by a fireball. Video of the aftermath showed that the entire Independence Square was covered in rubble, and the windows of several adjacent buildings were blown off.

Video: Russia attacks government building in central Kharkiv

In a video statement, Zelensky described the attack as “complete and unabashed terror.” “After this, Russia is a terror state,” he said. “No one will forgive. No one will forget.”

Satellite imagery also showed a convoy of 40-mile-long (64-kilometer)-long Russian armoured and support vehicles approaching Kyiv from the north, with the aim of bringing an overwhelming force to the Ukrainian capital of three million people .

“The situation in Kyiv remains tense,” Ukraine’s defense ministry said. “Russian occupiers are using tactics to destroy civilian infrastructure. . . Columns of Russian equipment continue to enter the outskirts of the capital.”

The lifting of a city-wide curfew on Monday led to long queues outside newly opened grocery stores and international agencies warned of a looming humanitarian crisis.

Ukraine’s railway service said on Tuesday it had added carriages to its Kyiv-Warsaw train to evacuate women and children from the capital. It added that arrangements had been made to provide evacuees with food and shelter once they were in Poland.

The deteriorating situation in many of Ukraine’s largest cities could accelerate the flight of civilians fleeing the conflict from the country. Reports from the Polish border showed some refugees had been waiting up to 60 hours to cross the border, the United Nations said on Tuesday.

Filippo Grandi, the UN High Commissioner for Refugees, said: “In nearly 40 years of working in refugee crises, I have rarely seen such an incredibly fast-growing exodus – since the Balkan Wars. The largest in Europe.”

French Finance Minister Bruno Le Maire has pledged to wage a “full-scale economic and financial war” against the country as tough Western sanctions grow on Russia, saying: “We will spark a The collapse of the Russian economy.”

His remarks were scorned by Moscow. “Watch your tongue, gentlemen!” tweeted Dmitry Medvedev, a senior security official and former Russian president. “Don’t forget that economic wars have often turned into real wars throughout human history.”

Kremlin spokesman Dmitry Peskov said the sanctions would not make the country change the course of its so-called special military operations. “None of us will change our usual position,” Peskov told reporters.

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Using a series of statements, Zelensky accused Russia of violating the laws of war by launching indiscriminate attacks, adding that since the invasion began, Ukraine had suffered 56 missile strikes and Russia had fired 113 more cruise missiles.

“A peaceful city. A peaceful residential area. No military targets,” Zelensky said of the attack on Kharkiv. “The Russians know where they’re shooting.”

Zelensky, unshaven and clad in khakis, later addressed the European Parliament urging it to accept Ukraine’s membership of the European Union. “Make sure you’re with us. Make sure you’re a true European. Then life will triumph over darkness,” he said.