Russia's offensive in Ukraine appears to be the beginning of a "large-scale invasion" aimed at taking down the capital city of Kyiv, a senior U.S. defense official said Thursday.

The official described three "main axes of assault," including what looks to be a strong push toward the capital.

RUSSIA LAUNCHES INVASION OF UKRAINE: LIVE UPDATES

"It's our assessment that they have every intention of basically decapitating the government," the official said, specifying that "they're making a move on Kyiv."

The official said the invasion looks to be in an "initial phase" right now, with multiple phases to follow. What those future phases will look like, however, are difficult to predict, even though what has happened thus far has not been surprising.

PUTIN LAUNCHES ALL-OUT WAR ON UKRAINE IN LARGEST EUROPEAN ATTACK SINCE WWII

The official did predict that the war is expected to be "very bloody, very costly, and very impactful on European security writ large, perhaps for a long, long time to come."

The U.S. has observed Ukrainians fighting back, particularly in the city of Kharkiv, but also around the airport in Kyiv. The Ukrainian military claims that they have taken down a number of Russian jets, but the official could not confirm this.

Russian President Vladimir Putin has threatened that any country that intervenes will face repercussions "never seen in history," leading to speculation that this was a threat of a nuclear response. The senior defense official said that while the U.S. does not know in "perfect detail" what Russia is planning, they "don't see an increased threat in that regard."

Russia's Ukraine attacks reported

The map shows where Russia attacks been reported in Ukraine

The U.S. is sending military assistance to the region, but will not be entering Ukraine itself. While F-35 aircrafts are being sent to Estonia, Romania, and Lithuania, expected to arrive later on Thursday, the Biden administration has repeatedly insisted that U.S. troops will not be fighting in Ukraine.

CLICK HERE TO GET THE FOX NEWS APP

Similarly, on Thursday NATO Secretary General Jens Stoltenberg described defense forces that are at the ready to defend NATO members if necessary, but there has been no talk of entering Ukraine, which is not part of NATO.