Street fighting broke out in Ukraine’s second-largest city Sunday and Russian troops put increasing pressure on strategic ports in the country’s south following a wave of attacks on airfields and fuel facilities elsewhere that appeared to mark a new phase of Russia’s invasion.

Following its gains on the ground, Russia sent a delegation to Belarus for peace talks with Ukraine, according to the Kremlin. Ukraine’s president suggested other locations, saying his country was unwilling to meet in Belarus because it served as a staging ground for the invasion.

Until Sunday, Russia’s troops had remained on the outskirts of Kharkiv, a city of 1.4 million about 20 kilometers (12.4 miles) south of the border with Russia, while other forces rolled past to press the offensive deeper into Ukraine.

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Videos posted on Ukrainian media and social networks showed Russian vehicles moving across Kharkiv and Russian troops roaming the city in small groups. One video showed Ukrainian soldiers inspecting Russian light utility vehicles damaged by shelling and abandoned by Russian troops on a street.

Oleh Sinehubov, the head of the Kharkiv regional administration, told civilians not to leave their homes.

As Russia pushes ahead with its offensive, the West is working to equip the outnumbered Ukrainian forces with weapons and ammunition while punishing Russia with far-reaching sanctions intended to further isolate Moscow.

Huge explosions lit up the sky early Sunday near the capital, Kyiv, where people hunkered down in homes, underground garages and subway stations in anticipation of a full-scale assault by Russian forces.

Flames billowed into the sky before dawn from an oil depot near an air base in Vasylkiv, where there has been intense fighting, according to the town’s mayor. President Volodymyr Zelenskyy’s office said another explosion was at the civilian Zhuliany airport.

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Zelenskyy’s office also said Russian forces blew up a gas pipeline in Kharkiv, prompting the government to warn people to protect themselves from the smoke by covering their windows with damp cloth or gauze.

“We will fight for as long as needed to liberate our country,” Zelenskyy said.

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 neighboring countries, and the United Nations warned the number could grow to 4 million if fighting escalates.

Russia’s military also put increasing pressure on strategic ports in the south of Ukraine, blocking the strategic cities of Kherson on the Black Sea and the port of Berdyansk on the Azov Sea, a Russian Defense Ministry spokesman, Maj. Gen. Igor Konashenkov, said.

He said the Russian forces also have taken control of an air base near Kherson and the city of Henichesk on the Azov Sea. Ukrainian authorities previously have reported fighting in various areas along the coast.

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As Russia has closed in on the Ukrainian capital, it has also focused on pressing its offensive in the country’s south in an apparent bid to take control of its coast stretching from the border with Romania in the west to the border with Russia in the east.

The Ukrainian authorities have reported fighting going on near Odesa, Mykolaiv and other areas.

Russia’s advances along Ukraine’s coast mark an attempt to cut the country’s access to its sea ports that would deal a major blow to its economy. The offensive in the south also could allow Moscow to build a land corridor to Crimea, which until now was connected to Russia by a 19-kilometer (12-mile) bridge, the longest bridge in Europe which opened in 2018.

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.

Kremlin spokesman Dmitry Peskov said a Russian delegation of military officials and diplomats had arrived Sunday in the Belarusian city of Gomel for talks with Ukraine. Zelenskyy on Friday offered to negotiate a key Russian demand: abandoning ambitions of joining NATO.

“The Russian delegation is ready for talks and we are now waiting for the Ukrainians,”Peskov said.

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Ukraine’s president said his country was ready for peace talks with Russia but not in Belarus given the Russian ally’s role in the invasion. Speaking in a video message Sunday, Zelenskyy named Warsaw, Bratislava, Istanbul, Budapest or Baku as alternative venues.

Speaking in Russian, Zelenskyy said that Ukraine wants peace talks and will “accept any other city in a country that hasn’t been used for launching missiles. Only then, the talks could be honest and put an end to the war.”

To aid Ukraine’s ability to hold out, the U.S. pledged an additional $350 million in military assistance to Ukraine, including anti-tank weapons, body armor 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 U.S., 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.

Responding to a request from Ukraine’s minister of digital transformation, tech billionaire Elon Musk said on Twitter his satellite-based internet system Starlink was now active in Ukraine and that there were “more terminals en route.”

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

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A senior US defense official said Saturday that more than half the Russian combat power amassed along Ukraine’s borders had entered the country and Moscow has had to commit more fuel supply and other support units inside Ukraine than originally anticipated. The official spoke on condition of anonymity to discuss internal US assessments.

The curfew forcing everyone in Kyiv inside was set to last through Monday morning. The relative quiet of the capital was sporadically broken by gunfire.

Fighting on the city’s outskirts suggested that small Russian units were trying to clear a path for the main forces. Small groups of Russian troops were reported inside Kyiv, but Britain and the U.S. said the bulk of the forces were 19 miles (30 kilometers) from the city’s center as of Saturday afternoon.

Russia claims its assault on Ukraine is aimed only at military targets, but bridges, schools and residential neighborhoods have been hit.

Ukraine’s health minister reported Saturday that 198 people, including three children, had been killed and more than 1,000 others wounded during Europe’s largest land war since World War II. It was unclear whether those figures included both military and civilian casualties.

Ukraine’s ambassador to the US, Oksana Markarova, said troops in Kyiv were fighting Russian “sabotage groups.” Ukraine says some 200 Russian soldiers have been captured and thousands killed. Moscow has not provided casualty figures.

Markarova said Ukraine was gathering evidence of shelling of residential areas, kindergartens and hospitals to submit to The Hague as possible crimes against humanity.

Putin sent troops into Ukraine after denying for weeks that he intended to do so, all the while building up a force of almost 200,000 troops along the countries’ borders. He claims the West has failed to take seriously Russia’s security concerns about NATO, the Western military alliance that Ukraine aspires to join. But he has also expressed scorn about Ukraine’s right to exist as an independent state.

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