Reports of explosions and troops crossing border

(CNN) — Russian forces have begun an attack on Ukraine, with reports of troops crossing the border to the north and south, explosions in multiple cities including the capital Kyiv and Russian President Vladimir Putin warning of bloodshed unless Ukrainian forces lay down their arms.

Reports of military action came as world leaders attempted to avert catastrophe with a meeting of the United Nations Security Council amid warnings that a possible invasion could start the biggest war in Europe since 1945.

The attack prompted immediate condemnation from the United States and its allies, which have threatened to enact “full scale” sanctions in response to Russian military aggression.

As the sound of air raid sirens rang out across the Ukrainian capital early Thursday, the country’s President Volodymyr Zelensky moved to enact martial law and government ministers accused Moscow of launching a “full scale invasion.” But as dawn broke in Kyiv, there was still traffic on the streets and in Kramatorsk a local official said people were calm and going to work.

Ukraine Foreign Minister Dmytro Kuleba tweeted on his official page that “peaceful Ukrainian cities are under strike” and called the situation “a war of aggression.” And, an adviser for the Ukraine Interior Minister, Anton Gerashchenko, told journalists Russia’s “invasion has begun” with “missile strikes on Kyiv,” the capital.

Gerashchenko said troops had landed in the city of Odessa and were crossing the border in Kharkiv, the second-largest city in Ukraine located in the country’s northeast. In a post on his Facebook page, he also said control centers such as airfields and military headquarters were being hit by shelling in Kyiv and Kharkiv, and that there was artillery fire along the border.

CNN witnessed, through a livestream video, troops atop a column of military vehicles entering Ukraine from a border crossing with Belarus. The livestream video was taken at the Senkivka, Urkaine crossing with Veselovka, Belarus. The column was seen entering Ukraine around 6:48 a.m. local time.

In recent weeks, Russia has amassed a significant number of troops, vehicles and tanks in Belarus near the border with Ukraine. During that time, the two countries have held joint military exercises across the countries, and near the Belarus-Ukrainian border.

CNN teams in Russia and Ukraine also heard blasts from several parts of Ukraine, including near the capital Kyiv and the city of Odessa, and images released by the office of President Zelensky showed large explosions to the east of the capital Kyiv with huge columns of smoke rising into the air.

Hours before, Putin announced a military operation in the Donbas region of eastern Ukraine, which contains the separatist-held regions of Donetsk and Luhansk which Moscow recognized as independent on Monday.

In the address, broadcast live on Russian national television, Putin urged Ukrainian forces to lay down their arms and go home, saying all responsibility for possible bloodshed will be entirely on the conscience of the Ukrainian government.

But he added: “Our plans are not to occupy Ukraine, we do not plan to impose ourselves on anyone.”

Before the announcement of military action, Zelensky appealed for peace but vowed the country would defend itself.

“We have no need for another Cold war, or a bloody war, or a hybrid war,” Zelensky said. “But if we are attacked militarily, if they try to take away our freedom, our lives, our children’s’ lives, we will defend ourselves,” the Ukrainian President added, in a speech in Russian and directed at Russian citizens.”When you attack, you will see our faces and not our spines, our faces.”

US President Joe Biden issued a statement saying Russia had launched “an unprovoked and unjustified attack” on the Ukrainian people.

“President Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering,” he said. “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.”

Reports of explosions

A CNN team in the capital Kyiv reported explosions coming from the direction of the international airport, around 15 miles (24 kilometers) east of the center of Kyiv. Social media accounts reported several explosions in the Boryspil area to the east of the capital, where the international airport is located.

CNN has not confirmed that the airport has been targeted.

A CNN team in Kharkiv, the second-largest city in Ukraine located in the country’s northeast, heard a “steady stream of loud explosions.”

A team in Odessa heard two groups of explosions about 20 minutes apart, and another team in southeastern Zaporizhzhia, which lies on the Dnieper River that cuts through the country, heard at least one very distant explosion. Two people in Kramatorsk told CNN that they have heard at least two massive explosions in the early hours of Thursday.

Before Putin’s address, Russia issued a notice banning civil aircraft from flight routes bordering north-eastern Ukraine. Separately, Ukrainian aviation authorities issued a notice restricting Ukrainian airspace in the regions around the capital Kyiv, Dnipro, Lviv, Odessa and Simferopol.

European aviation regulators said there is a “high risk” to civilian aircraft near the Ukrainian border, fearing they could be targeted on purpose at any altitude.

“Air operators are reminded that this is now an active conflict zone,” said the European Union Aviation Safety Agency, known as EASA.

In an apparent escalation in cyberattacks on Ukrainian infrastructure, several key Ukrainian websites were down as of 3 a.m local time on Thursday (8 p.m. ET Wednesday), including those of Ukraine’s Foreign Ministry, the Interior Ministry, the Security Service in Ukraine, the Cabinet of Ministers, the Ministry of Infrastructure, and the Ministry of Justice.

Military operation comes as UN Security Council meets

The military operation came as the UN Security Council had “gathered in the council seeking peace” Wednesday night.

US Ambassador to the UN Linda Thomas-Greenfield said Russian President Vladimir Putin had “delivered a message of war.”

“This is a grave emergency. The council will need to act,” she said. “We will put a resolution on the table tomorrow.”

Her Ukrainian counterpart Sergiy Kyslytsya begged the Security Council to help stop a war, saying it was too late to speak about de-escalation.

“It is the responsibility of this body to stop the war,” Kyslytsya said. “So I call on every one of you to do everything possible to stop the war,” he added, looking around the room.

Permanent Representative of the Russian Federation to the UN Vasily Nebenzya said the aim of Russia’s military operation in Donbas is to protect locals who have been “suffering genocide” at the hands of the Ukrainian government for over eight years.

“The route of today’s crisis around Ukraine is the actions of Ukraine itself, who for many years were sabotaging its obligations under the Minsk package of measures,” said Nebenzya.