• Russia launched an attack on Ukraine early Thursday local time, capping a months long buildup of forces.
• Explosions were heard in Kyiv, Ukraine's capital, according to NBC News.
• U.S. President Joe Biden said he would announce more retaliatory measures Thursday in response to the attack.

WASHINGTON – Russian forces launched an attack on Ukraine early Thursday morning local time, marking the devastating culmination of months of a Kremlin military buildup near its former Soviet neighbor's border.

After Russian President Vladimir Putin announced a military action in Ukraine, explosions were heard in the country's capital of Kyiv, according to NBC News.

U.S. President Joe Biden, who had worked to deter an invasion through diplomacy and a round of sanctions, condemned the action Wednesday night.

In a statement, Biden said Putin "has chosen a premeditated war that will bring a catastrophic loss of life and human suffering." He said he would meet with G-7 leaders Thursday then "announce the further consequences the United States and our Allies and partners will impose on Russia for this needless act of aggression against Ukraine and global peace and security."

The invasion caps a drawn out buildup to an attack on Ukraine foreshadowed by Putin. It came even as Europe and the U.S. made last-ditch attempts at diplomacy during an emergency United Nations meeting Wednesday night.

A Pentagon official said earlier on Wednesday that about 80% of Russia's military forces amassed around Ukraine were "ready to go" for an attack.

"They are literally ready to go now if they get the order to go," the official, who spoke on the condition of anonymity, added.

The official said that Russian Putin has assembled enough aviation, maritime and ground assets to "conduct a large-scale invasion." 

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.

For months, the U.S. and its Western allies have watched a steady buildup of Kremlin forces along Ukraine's border with Russia and Belarus. The increased military presence mimics Russia's playbook ahead of its 2014 illegal annexation of Crimea, a peninsula on the Black Sea, which sparked international uproar and triggered sanctions against Moscow.

The Kremlin has denied that the extraordinary deployment of Russian troops outfitted with advanced military equipment along Ukraine's borders are preparing for an invasion.

The invasion follows Putin's stunning decision to recognize two breakaway regions of eastern Ukraine.

This story is developing. Please check back for updates.