• Shortly after Putin spoke in a special televised address on Russian state TV, explosions could be heard in the pre-dawn quiet of the Ukrainian capital of Kyiv.
• Gunfire rattled near the capital's main airport, the Interfax news agency said.
Russian President Vladimir Putin authorised a military operation in eastern Ukraine on Thursday in what appeared to be the start of war in Europe over Russia's demands for an end to NATO's eastward expansion.
Shortly after Putin spoke in a special televised address on Russian state TV, explosions could be heard in the pre-dawn quiet of the Ukrainian capital of Kyiv. Gunfire rattled near the capital's main airport, the Interfax news agency said.
Explosions also rocked the breakaway eastern Ukrainian city of Donetsk and civilian aircraft were warned away as the United States said a major attack by Russia on its neighbour was imminent.
Putin said he had authorised a special military operation in breakaway areas of eastern Ukraine and clashes between Russian and Ukrainian forces were only a matter of time.
Hours earlier, pro-Russian separatists issued a plea to Moscow for help to stop alleged Ukrainian aggression - claims the United States dismissed as Russian propaganda.
Putin said he had ordered Russian forces to protect the people and demanded Ukrainian forces lay down their arms.
"All responsibility for bloodshed will be on the conscience of the ruling regime in Ukraine," Putin said.
He repeated his position that NATO expansion to include Ukraine was unacceptable and said Russia had been left with no choice but to defend itself against what he said were threats emanating from Ukraine.
The scope of the Russian military operation was not immediately clear. Moscow has long denied that it has plans to invade despite massing tens of thousands of troops near its neighbour.
Ukrainian President Volodymyr Zelenskiy said on Wednesday that Moscow had approved an offensive and had not replied to an invitation for talks.
"Today I initiated a telephone conversation with the President of the Russian Federation. The result was silence," he said.
U.S. President Joe Biden said his prayers were with the people of Ukraine "as they suffer an unprovoked and unjustified attack by Russian military forces".
"President Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering. Russia alone is responsible for the death and destruction this attack will bring, and the United States and its Allies and partners will respond in a united and decisive way. The world will hold Russia accountable."
He said he would announce further sanctions on Russia on Thursday, in addition to financial measures imposed this week.
The Russian operations began as the U.N. Security Council held an emergency meeting on the Ukraine crisis in New York.
A draft resolution calling out Moscow over its actions toward its neighbour is doomed to fail due to Russia veto power, however a Security Council diplomat said it would put Russia on notice that it is "not in compliance with international law."
Ukraine restricted civilian flights in its airspace due to "potential hazard", hours after a conflict zone monitor warned airlines should stop overflights over the risk of an unintended shooting down or cyber attack.
An El Al flight from Tel Aviv to Toronto and a LOT Polish Airlines flight from Warsaw to Kyiv turned out of Ukraine's airspace around the time a notice was issued, according to flight tracking website FlightRadar24.
Russia also partially closed its airspace in the Rostov flight information region to the east of its border with Ukraine "in order to provide safety" for civil aviation flights, according to its notice to airmen.
Convoys of military equipment including nine tanks were seen moving towards Donetsk earlier on Wednesday from the direction of the Russian border, a Reuters witness reported.
Shelling has intensified since Monday when Putin recognised two separatist regions as independent and ordered the deployment of what he called peacekeepers, a move the West calls the start of an invasion.
U.S. Treasury Secretary Janet Yellen spoke separately with her counterparts from Britain and Canada, while G7 leaders are also due to talk on Thursday.
Satellite imagery taken on Wednesday showed new deployments in western Russia, many of them within 10 miles (16 km) of the border with Ukraine and less than 50 miles from the Ukrainian city of Kharkiv, U.S. satellite company Maxar said.
The images showed field deployment, military convoys, artillery and armoured personnel carriers with support equipment and troops. The images could not be independently verified by Reuters.
A 30-day state of emergency in Ukraine restricting the freedom of movement of conscripted reservists, curbing the media and imposing personal document checks, according to a draft text, begins on Thursday.
The Ukrainian government has also announced compulsory military service for all men of fighting age.
Western countries and Japan imposed sanctions on Russian banks and individuals but have held off their toughest measures until an invasion began.
The United States stepped up the pressure on Wednesday by imposing penalties on the Russian firm building the Nord Stream 2 gas pipeline and its corporate officers.
Germany on Tuesday froze approvals for the pipeline, which has been built but was not yet in operation, amid concerns it could allow Moscow to weaponize energy supplies to Europe.