(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 warnings from Russian President Vladimir Putin of bloodshed unless Ukrainian forces lay down their arms.

The offensive that Russia called a "special military operation" 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 first blasts were heard at around 5 a.m. Thursday, prompting immediate condemnation from the United States and its allies, which have threatened to enact "full scale" sanctions in response to Russian military aggression.

As air raid sirens rang out across Kyiv early Thursday, Ukrainian President Volodymyr Zelensky moved to enact martial law and government ministers accused Moscow of launching a "full scale invasion."As dawn broke, heavy traffic could be seen clogging roads heading west out of the city, while further east, near the Russian border, the mayor of Kharkiv urged citizens not to leave their homes.

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.

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, Ukraine 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.

The General Staff of the Ukrainian Armed Forces said five Russian aircraft and a helicopter were shot down early Thursday, claims Russia denied via state news agency TASS.

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 violation of international law.

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.

"Our plans are not to occupy Ukraine, we do not plan to impose ourselves on anyone," he said, but threatened "those who may be tempted to intervene" on Ukraine's behalf.

"Whoever tries to interfere with us, and even more so to create threats to our country, to our people, should know that Russia's response will be immediate and will lead you to such consequences as you have never experienced in your history," he said.

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

"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 said, 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."

EU Commission President Ursula von der Leyen promised to "weaken Russia's economic base and its capacity to modernize" following its "barbaric attack" against Ukraine.

"We will freeze Russian assets in the European Union and stop the access of Russian banks to European financial markets," she said.

What's happening on the ground

Ukraine's borders were reportedly under attack from Russian forces to the north in Belarus and the south from Crimea, according to the Ukrainian State Border Service.

Russian troops had attacked border units, patrol border and checkpoints with "artillery, heavy equipment and small arms," the border service said.

The Russian military claimed Ukraine's border service "did not provide any resistance" and claimed to have "suppressed" Ukraine's air defenses. CNN was not able to immediately verify either of those claims.

CNN teams on the ground heard explosions in and near multiple Ukrainian cities, including Kyiv, the second largest city Kharkiv, Odessa, and distant firing from Zaporizhzhiya.

Ukraine Interior Minister adviser Gerashchenko tweeted that troops had landed in the southern city of Odessa and were crossing the border in Kharkiv, 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.

The Russian military released a statement Thursday claiming it was not targeting Ukrainian cities, saying "the civilian population is not at risk."

"The Russian Armed Forces are not launching any missile or artillery strikes on the cities of Ukraine. High-precision weapons destroy military infrastructure: military airfields, aviation, air defense facilities of the Armed Forces of Ukraine," the statement read.

But despite the Russian military's claims civilians wouldn't be targeted, many chose to leave the capital. Kyiv residents had previously been told to stay at home and pack a bag with necessities, in case they need to leave abruptly.

Witness told CNN subway stations had become improvised bunkers and they were full of people, with supplies organized in groups.

Across the city, red arrows painted on walls indicate the locations of the nearest bomb shelters. The city first put them up after the war broke out in the east of the country in 2014, but they were recently repainted again.

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

European aviation regulators warned civilian aircraft were at "high risk" near the Ukrainian border. "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 launch of the military operation came as members of the UN Security Council met in New York to call for a peaceful resolution to the impending crisis.

In ordering troops to enter Ukraine, 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 was 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.

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