The United States believes Russia's invasion of Ukraine is designed to decapitate Ukraine's government and one of the three main axes of assault is directed at the capital Kyiv, a senior U.S. defence official said on Thursday.

"The indications we've seen thus far, in just these first, not even 12 hours, are in keeping with our assessment earlier, that would be his goal: to decapitate this government," said the official, speaking on the condition of anonymity.

The official did not provide evidence.

The official said this appeared to be the initial phase of a large-scale Russian invasion and included more than 100 missiles launched by Russia, including cruise missiles, surface-to-air- missiles and those launched from the sea.

There had been no indication of any amphibious assaults yet by Russian forces but they used about 75 military aircraft in the first phase of the invasion, the official added.

"The targets, thus far... have been primarily focused on military and air defence. So barracks, ammunition warehouses, nearly 10 airfields targeted," the official said, adding the United States could not yet determine casualties or damage.