The Russian military opened its attack with around 100 missile launches in the first two hours, mainly targeting military infrastructure facilities, as well as sorties by 75 heavy and medium bombers, the official said.

The initial phase is focused on key cities, and the Pentagon expects the Russians to move on Kyiv, according to the official, speaking on grounds of anonymity.

"They have every intention of basically decapitating the government and installing their own means of governance," the official said.

The official said that Russian troops had crossed the border on the ground but gave no estimate on numbers.

"What we're seeing are initial phases of a large scale invasion."

"We haven't seen a conventional move like this, nation state to nation state, since World War II, certainly nothing on this size and scope and scale," the official said.

The invasion began on three axes aimed at seizing population centers.

The first involves troops entering from Russian-controlled Crimea toward the city of Kherson in the south.

The second is in incursion from Belarus into north-central Ukraine, toward Kyiv. And the third is in the northeast, a push from Russia near Belgorod aimed at the Ukrainian city of Kharkiv.

"The heaviest fighting we've seen so far is in Kharkiv," the official said.

The opening phase included ground and sea-based launches of short and medium-range ballistic missiles, cruise missiles and surface-to-air missiles.

Those, along with bomb attacks by aircraft, focused on airfields, barracks, and ammunition depots.

So far however, the Russians have not entered western Ukraine, and there have been "no indications" of an amphibious assault in the south from the Black Sea, the official said.

Nor have they targeted the Ukraine military's command and control facilities, or public communications.

"Public means of communication and media are accessible and are still active," the official said.

There were no estimates of the damages to Ukraine's military.

"We have seen indications that they are resisting and fighting back," the official said.

The Pentagon had no confirmation of Ukraine claims of having downed several Russian aircraft.