KYIV, Ukraine: Huge explosions lit up the predawn sky south of Kyiv early Sunday as Russia"s invasion of Ukraine hits its fourth day. President Volodymyr Zelenskyy’s office said one of the blasts was near the Zhuliany airport. It saud that Russian forces also blew up a gas pipeline in Kharkiv, the country’s second-largest city. Ukraine"s state service of special communications and information protection and the mayor of Vasylkiv, about 25 miles (40 kilometers) south of the capital, also said that an oil depot was hit. A mushroom-shaped explosion was shown in a video it posted on the Telegram messaging app. It was not immediately clear how important the pipeline was and whether the blast could disrupt gas shipments outside the city or the country. Despite the war, Ukraine continues to ship Russian natural gas to Europe. 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. Ukrainian forces had 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 neighboring countries, and the United Nations warned the number could grow to 4 million if fighting escalates. “We will fight for as long as needed to liberate our country,” Zelenskyy 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. 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 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 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. 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.” A senior US defense official said more than half the Russian combat power that was massed along Ukraine’s borders has 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 US said the bulk of the forces were 19 miles (30 kilometers) from the city’s center as of the afternoon. Russia claims its assault on Ukraine from the north, east and south 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. A missile struck a high-rise apartment building in Kyiv’s southwestern outskirts near one of the city’s two passenger airports, leaving a jagged hole of ravaged apartments over several floors. A rescue worker said six civilians were injured. 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. Markarova said Ukraine was gathering evidence of shelling of residential areas, kindergartens and hospitals to submit to The Hague as possible crimes against humanity. Zelenskyy reiterated his openness to talks with Russia in a video message, saying he welcomed an offer from Turkey and Azerbaijan to organize diplomatic efforts, which so far have faltered. The Kremlin confirmed a phone call between Putin and Azerbaijani President Ilham Aliyev but gave no hint of restarting talks. A day earlier, Zelenskyy offered to negotiate a key Russian demand: abandoning ambitions of joining NATO. 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.