Russian forces have attacked Ukraine after President Vladimir Putin announced he had authorised a "special military operation” in the country’s east at the same time as the United Nations Security Council met for its second emergency meeting this week.

Russian media on Thursday reported President Putin saying he had approved a "special military operation” in the Donbas region of Ukraine, where Moscow earlier recognised rebel-held territories in Luhansk and Donetsk and said they had asked for its "help”. Clashes between Russian and Ukrainian forces were only a matter of time, he added.

Shortly after Putin spoke, Al Jazeera’s Andrew Simmons, who is in Kyiv, said there were explosions in the capital and power had been cut.

It appeared to be a "full-scale attack”, targeting the airport and key buildings, he said. There was "chaos” in the city centre, he added.

Explosions also rocked the breakaway eastern Ukrainian city of Donetsk and civilian aircraft were warned away, while there were reports of naval landings at Odesa in Mariupol.

Ukrainian President Volodymyr Zelenskyy made a brief national address to say that he was declaring martial law and that Ukraine would "win”.

"Putin has just launched a full-scale invasion of Ukraine,” Ukraine’s Foreign Minister Dmytro Kuleba wrote on Twitter. "Peaceful Ukrainian cities are under strikes. This is a war of aggression. Ukraine will defend itself and will win. The world can and must stop Putin. The time to act is now.”

US President Joe Biden, in a phone call with Zelenskyy, condemned the "unprovoked and unjustified attack by Russian military forces”, the White House said.

Biden said in a statement he briefed Zelenskyy on Washington’s and its allies’ planned next steps against Russia, including "severe sanctions”.

In an impassioned appeal, UN chief Antonio Guterres condemned Russia’s actions.

"In the name of humanity, bring your troops back to Russia,” he said. "In the name of humanity, do not allow a war to start in Europe which could be the worst war since the beginning of the century with consequences not only devastating for Ukraine, not only tragic for the Russian Federation but with an impact we cannot even foresee.”

NATO called an emergency meeting with chief Jens Stoltenberg condemning what he said was a "reckless attack”.

‘You declared war’

Russia’s UN ambassador Vasily Nebenzya, who is the current president of the Security Council, admitted to fellow members of the council that "special operations” were under way, insisting to the other ambassadors that the action did not amount to war and was a consequence of Ukraine’s actions.

"The aim of the operation is to protect the people who for eight years have been suffering "genocide” of the Ukrainian regime,” he said, claiming the action was justified under Article 51 of the UN Charter (the article allows for individual or collective self-defence in the face of an armed attack on a UN member state).

Putin demanded Ukrainian forces lay down their arms, and repeated his position that any Ukrainian membership of NATO was unacceptable to Moscow, according to state media.

In the UN council chamber, Ukraine’s UN Ambassador Sergiy Kyslytsya appeared lost for words: "Three minutes ago, your president declared war on my country,” he said to Nebenzya.

In a second round of comments, having effectively called for Russia’s expulsion from the UN, he concluded: "There’s no purgatory for war criminals, Mr Ambassador, they go to hell”.

The US said it was calling for an urgent resolution at the Security Council, and the call was backed by the United Kingdom, France and Ireland who noted Russia’s utter "disdain” for the United Nations by its military assault. The meeting is expected within 24 hours.

As news of the assault on Ukraine filtered through, some diplomats condemned Russia.

Germany said it was a "shameless breach of international law” while the UK said the attack was "unprovoked, unjust” and it was a "dark day for Ukraine”.

The Russian action came almost at the same time as Linda Thomas-Greenfield, the United States ambassador to the UN, told the council that the US was concerned a "full-scale” invasion of Ukraine by Russia was imminent.

"Tonight, we’re seeing the Russians close airspace, move troops into Donbas and move forces into combat-ready positions,” she said.

"This is a perilous moment and we are here for one reason and one reason only – to ask Russia to stop.”

Analysts said it could be that Russia was beginning with a "shock and awe” attack in the belief that it might mean it could get what it wants without having to mount a larger scale attack.

It insisted the attacks were focused on military targets.

"Putin might think that will cause such shock that the government will collapse, that there will be calls for surrender,” William Courtney, an expert at the Rand Corporation in Washington, DC, told Al Jazeera. "If so, the Kremlin has probably underestimated the resilience of Ukrainians.”