Russian President Vladimir Putin watches with binoculars the Tsentr-2019 military exercise at the Donguz range near Orenburg city on September 20, 2019.Russian President Vladimir Putin watches with binoculars the Tsentr-2019 military exercise at the Donguz range near Orenburg city on September 20, 2019.

Russian forces launched an attack on Ukraine early Thursday morning local time, marking the culmination of months of a Kremlin military build up near its former Soviet neighbor.

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

The action Wednesday night was condemned by the U.S. President Joe Biden, who had worked to deter an invasion through diplomacy and sanctions.

In a statement, Biden said that Putin had chosen a war that would bring a catastrophic loss of life and human suffering.

The invasion caps a drawn out preparation for an attack on Ukraine. Even as Europe and the U.S. made last-ditch attempts at diplomacy, it came.

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

The official, who spoke on the condition of anonymity, said that they are ready to go if the order is given.

The official said that Russian Putin has enough assets to conduct a large-scale invasion.

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

According to our estimates, if Russia continues down this path, it could lead to a new refugee crisis with as many as 5 million more people displaced by Russia's war of choice.

The U.S. and its Western allies have been watching a steady build up of Russian forces along the border with Ukraine. The increased military presence is similar to Russia's annexation of the Black Sea peninsula of Crimea, which caused international uproar and triggered sanctions against Moscow.

The deployment of Russian troops with advanced military equipment along the borders of Ukraine has been denied by the Kremlin.

There is a developing story. You can check back for updates.