There are movements of Russian military and special forces coming into Ukraine from many directions, according to the officials: from the northeast via Russia; from the south via Moscow-annexed Crimea; and from the north via both Belarus and Russia.

U.S. intelligence believe these three axes were "designed to take key population centers" and that the early moves from the north toward Kyiv indicate an intention to remove the Ukrainian government, a senior U.S. defense official told reporters at the Pentagon.

"What we're seeing are initial phases of a large-scale invasion," the official said.

The initial attack included an estimate of more than 100 Russian-launched missiles -- mostly short-range ballistic missiles but also some medium-range ones -- and about 75 fixed-wing heavy and medium bombers. So far, the targets are mostly Ukrainian military infrastructure and air defense systems, the official said, adding that U.S. intelligence does not yet have a good sense of total damages or casualties.

The official could not give an exact estimate of how many Russian troops have crossed into Ukraine thus far but said that, at this early stage, it is certainly a minority of the 150,000 troops that were massed near the borders.

U.S. intelligence have seen indications that Ukrainian troops "are resisting and fighting back," the official said. Some fighting has been seen around the airport in Kyiv. But the heaviest fighting is currently occurring in the northeastern city of Kharkiv, some 300 miles east of Kyiv, according to the official.

"We have not seen the Russians thus far move into the western part of Ukraine," the official said. "We don't know exactly where things are going to unfold."

The U.S. official said Russia has conducted "ground incursion from Belarus to the northwest of Kyiv, and we have seen at least some indications of air assault incursions into Kharkiv."

"So missile, long range fires, and then there has been some insertion of troops both from the air and on the ground in the north," the official summarized.

"We haven't seen a conventional move like this, nation state to nation state [in Europe], since World War II," the official said, "It has every potential to be very bloody, very costly and very impactful on European security writ large."

The official said he did not have a number on casualties.

-ABC News' Matt Seyler

Ukraine has lost control of a key military airport that is located less than 20 miles from the center of the capital Kyiv, according to Ukraine’s deputy interior minister.

Russian special forces landed at the Hostomel airport just on the edge of Kyiv earlier Thursday and after fierce fighting the base remains in Russian hands, deputy minister Anton Gerashchenko said.

-ABC News' Patrick Reevell

The United Kingdom's Prime Minister, Boris Johnson, has announced a new package of sanctions in the House of Commons targeting over 100 Russian entities and individuals.

Johnson said this would totally exclude Russian banks from the U.K. financial system, adding, "oligarchs in London will have nowhere to hide."

He vowed, "We will continue on a relentless mission to squeeze Russia from the global economy” as Putin seeks to “redraw the map of Europe in blood.”

-ABC News' Guy Davies

All U.S. diplomats from the mission to Ukraine are in Poland and will remain there, with no plans to travel to Ukraine for now, the State Department confirmed.

"We will continually assess the security situation to determine when it may be safe for U.S. government personnel to return to Ukraine to conduct diplomacy on the ground and provide in-person consular services," a State Department spokesperson told ABC News.

-ABC News' Conor Finnegan