Police investigate the consequences of Russian shelling in Kyiv, Ukraine. Photo / AP
Ukraine's Foreign Minister Dmytro Kuleba has called on the world to immediately respond to Russia's invasion of the embattled nation.
On social media, Kuleba shared a pointed message to the world to ignore claims by Russia that the so-called "military operation" is anything other than a war.
He outlined five key demands for world superpowers such as the United States and Britain, which have spent recent weeks pledging support for the country now under Russian President Vladimir Putin's gaze.
"The world must act immediately," Kuleba said in a series of tweets as bombs exploded in Kyiv. "The Future of Europe and the world is at stake."
Putin has just launched a full-scale invasion of Ukraine. 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.
Ukraine's leader, President Volodymyr Zelenskyy, said he had unsuccessfully sought talks with Putin as the crisis between their two countries entered its most dangerous phase so far.
"I initiated a telephone call with the President of the Russian Federation. Result: Silence," Zelenskyy said in a late-night address.
Zelenskyy issued a statement to the people of Ukraine shortly after 7am local time.
"Dear citizens, please do not leave your homes today," he said.
"Russia conducted airstrikes on military infrastructure and border guards. There were blasts heard in multiple cities."
He said Ukraine was introducing "martial law on the whole territory of the country".
"Stay calm, stay at home if you can. No panic. We are strong. We are ready for everything. We will win over everybody because we are Ukraine."
Putin made a surprise statement on television to declare his intentions. "I have made the decision of a military operation," he said shortly before 6am local time in Moscow, as he vowed retaliation against anyone who interfered.
He also called on the Ukraine military to lay down its arms. His statement came after the Kremlin said rebel leaders in eastern Ukraine had asked Moscow for military help against Kyiv.
Photos of the initial wreckage began filtering through world media on Thursday evening.
Putin has defied a barrage of international criticism over the crisis, with some Western leaders saying he was no longer rational.
His announcement of the military operation came ahead of a last-ditch summit involving European Union leaders in Brussels planned for Thursday.
The 27-nation bloc had also imposed sanctions on Russia's Defence Minister, Sergei Shoigu, and high-ranking figures including the commanders of Russia's Army, Navy and Air Force, another part of the wave of Western punishment after Putin sought to rewrite Ukraine's borders.
The United Nations Security Council met late on Wednesday for its second emergency session in three days over the crisis, with a personal plea by UN Secretary-General Antonio Guterres to Putin going unheeded.
"President Putin, stop your troops from attacking Ukraine, give peace a chance, too many people have already died," Guterres said.
The US ambassador to the United Nations, Linda Thomas-Greenfield, warned that an all-out Russian invasion could displace five million people, triggering a new European refugee crisis.
Before Putin's announcement, Ukraine had urged its approximately three million citizens living in Russia to leave.
"We are united in believing that the future of European security is being decided right now, here in our home, in Ukraine," President Zelenskyy said during a joint media appearance with the visiting leaders of Poland and Lithuania.
Western capitals said Russia had amassed 150,000 troops in combat formations on Ukraine's borders with Russia, Belarus and Russian-occupied Crimea and on warships in the Black Sea.
Ukraine has around 200,000 military personnel and Wednesday's call up could see up to 250,000 reservists aged between 18 and 60 receive their mobilisation papers.