A senior US Defense Department official says Russia's attacks on Ukraine are an initial phase of a large-scale invasion.

The official said on Thursday that fierce missile attacks began at 4:30 a.m., local time.

The official said Russia fired more than 160 short- and intermediate-range ballistic missiles, as well as cruise missiles, from land and the Black Sea. About 75 aircraft, including bombers, were also used.

The official also said the main targets were military and air defense facilities such as ammunition warehouses and airfields, adding that the extent of damage and the number of military and civilian casualties are unknown.

The Pentagon official said that after the missile attacks, Russian ground troops began invading Ukrainian territory at around noon on Thursday.

The official said the troops are advancing in three directions: from neighboring Belarus to the capital Kyiv; from the northeast border to the second-largest city Kharkiv; and from Crimea to the city of Kherson.

The route to Kyiv has been divided in two -- one to the capital's northeast and the other to the northwest.

The defense official said the Russian military is believed to be trying to take control of densely populated areas, adding that it has yet to deploy most of its troops.