Earlier, Vladimir Putin had ordered his military to put Russia’s nuclear deterrence forces on high alert in response to “aggressive statements” by NATO countries

The Russian army on Sunday admitted that there were "killed and injured" soldiers among its troops in Ukraine on the fourth day of its invasion of the country, without specifying how many Russians had died there.

"Russian servicemen are showing courage and heroism while fulfilling combat tasks in the special military operation. Unfortunately, there are killed and injured among our comrades," Moscow's army spokesman Igor Konashenkov said on state television.

President Volodymyr Zelensky said Sunday Ukraine was willing to hold talks with Russia, but rejected convening them in neighbouring Belarus as it was being used as a launchpad for Moscow's invasion.

The Russian government has said that it is ready for talks with Ukraine in Belarus, news agency AFP reported on Sunday.

Russian troops have blown up a natural gas pipeline in the Ukrainian city of Kharkiv, Ukraine’s state service of special communications and information protection has said.

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.

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

Zelenskyy'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.

"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 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.

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.”

A senior U.S. defence 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 U.S. 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 centre 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 neighbourhoods have been hit.