KYIV, Feb 23 - Russian forces attacked Ukraine early Thursday morning, launching an offensive that threatens to kill thousands of people, force millions more to flee, and destabilize much of Europe, with the consequences certain to reverberate across the world.

Blasts were heard from Kyiv, the capital, to the eastern city of Kharkiv — missile strikes, the Ukrainian interior ministry told CNN — with reports of outgoing artillery fire from Russian forces across the border, The Insider reported.

"The prayers of the entire world are with the people of Ukraine tonight as they suffer an unprovoked and unjustified attack by Russian military forces," US President Joe Biden said in a statement. "President Putin has chosen a pre-mediated war that will bring a catastrophic loss of life and human suffering."

Thursday's invasion was preceded by a formal request for military intervention from Russian-backed separatists in eastern Ukraine, where rebels and Russian forces have been fighting Ukrainian troops for eight years.

On the eve of the attack, Ukrainian President Volodymyr Zelensky took to the airwaves in a last-ditch appeal for peace — while pledging that Ukrainians would "fight back" against any further Russian incursion.

Hours later, in an early morning address that coincided with an emergency meeting of the United Nations Security Council, Russian President Vladimir Putin effectively declared war, insisting that Russian forces would strive for the "demilitarization" and "de-Nazification" of Ukraine, whose president is Jewish. Explosions were heard soon after in Kharkiv and Kyiv, CNN reported, with blasts also heard in the port city of Odessa.

The attack will be a "full scale" and "comprehensive military assault," Sen. Marco Rubio, vice chairman of the Senate Select Committee on Intelligence, wrote on Twitter. The Florida Republican said the invasion would include air strikes as well as amphibious landings, cyber attacks, and "a large ground force to occupy a large swath of territory."

Indeed, minutes later Reuters reported that an amphibious landing operation was taking place near Odessa on the Azov sea.

Earlier on Wednesday, U.S. Ambassador to the United Nations Linda Thomas-Greenfield warned that Russia’s aggression in Ukraine could lead to one of the largest refugee crises in the world.

“If Russia continues down this path, it could, according to our estimates create a new refugee crisis, one of the largest facing the world today, with as many as 5 million more people displaced by Russia’s war of choice,” Thomas-Greenfield said during an address at the United Nations.

Tensions had been mounting for months as Russia amassed troops, tanks, and amphibious ships near Ukraine's borders. Beginning in late 2021, Russia began amassing tens of thousands of troops on Ukraine's borders, with roughly 190,000 deployed by the time of the attack.

Russia in mid-February claimed to be withdrawing some of its troops from Ukraine's borders, but the US and NATO didn't buy it, citing intelligence that the Kremlin had actually deployed thousands more soldiers.

President Biden warned last week that he believes Russian forces will ultimately besiege Ukraine's capitol, which has a population of nearly 3 million people. Earlier this week, he unveiled a new round of sanctions against Russian officials in response to Moscow deploying troops to the eastern Donbas region, which he characterized "the beginning of a Russian invasion of Ukraine." (in/ez)