Ukrainian forces put up fierce resistance to slow the advance of the larger and more powerful Russian military closing in on the capital, as the US and EU rushed ammunition and weapons to Kyiv and announced powerful new financial sanctions aimed at further isolating Moscow.

Terrified men, women and children sought safety inside and underground, and the government maintained a 39-hour curfew to keep people off the streets. More than 150,000 Ukrainians fled for Poland, Moldova and other neighbouring countries, and the United Nations warned the number could grow to 4 million if fighting escalates.

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Huge explosions lit up the predawn sky south of Kyiv early Sunday. President Volodymyr Zelensky‘s office said one of the blasts was near the Zhuliany airport, and the mayor of Vasylkiv, about 25 miles (40 kilometres) south of the capital, said an oil depot was hit.

Zelensky’s office also said Russian forces blew up a gas pipeline in Kharkiv, the country’s second-largest city. The government warned that smoke from the huge explosion could cause an “environmental catastrophe” and advised people to cover their windows with damp cloth or gauze.

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“We will fight for as long as needed to liberate our country,” Zelensky vowed.

President Vladimir Putin hasn’t disclosed his ultimate plans, but Western officials believe he is determined to overthrow Ukraine’s government and replace it with a regime of his own, redrawing the map of Europe and reviving Moscow’s Cold War-era influence.

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To aid Ukraine’s ability to hold out, the US pledged an additional $350 million in military assistance to Ukraine, including anti-tank weapons, body armour and small arms. Germany said it would send missiles and anti-tank weapons to the besieged country and that it would close its airspace to Russian planes.

The US, European Union and United Kingdom agreed to block “selected” Russian banks from the SWIFT global financial messaging system, which moves money around more than 11,000 banks and other financial institutions worldwide, part of a new round of sanctions aiming to impose a severe cost on Moscow for the invasion. They also agreed to impose ”restrictive measures” on Russia’s central bank.

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It was unclear how much territory Russian forces had seized or to what extent their advance had been stalled. Britain’s Ministry of Defence said “the speed of the Russian advance has temporarily slowed, likely as a result of acute logistical difficulties and strong Ukrainian resistance.”