The Russian military opened its attack with around 100 ballistic missile launches 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 the capital 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.

"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.

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.

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.