After opening its invasion with more than 160 missile strikes and sorties by dozens of attack aircraft, Russian troops were moving southward from Belarus seeking to capture an airfield just north of the capital.

They have virtually eliminated the Ukraine air force's ability to fight back, leaving Kyiv increasingly vulnerable, the officials said.

"A lot will depend on the resistance that the Ukrainians can put up, but I'm confident that the Russians will in the coming hours seek to apply an overwhelming force on the capital," a Western intelligence official in Europe said.

"In just the intervening hours (since Thursday morning) they have gotten closer to Kyiv," a US defense official in Washington concurred.

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

- Three fronts -

The push from Belarus towards Kyiv was one of three fronts that Russia forces opened since launching the invasion early Thursday, the US official said.

An invasion force had also moved up from Crimea in the south towards the city of Kherson.

And a third axis of attack was in the northeast, marked by a push from Russia near Belgorod and aimed at capturing the Ukrainian city of Kharkiv.

"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 US official said.

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

The attack opened just before dawn early Thursday with a raft of missile launches, including short and medium-range ballistic missiles and cruise missiles, according to the US official.

Those were launched from both land and ships on the Black Sea, and targeted military infrastructure, including airports, barracks and munitions depots.

The Western intelligence official said Russian air power, with dozens of advanced fight-bombers and scores of attack helicopters, had overwhelmed Ukraine's defenses.

"Those air defences are now effectively eliminated. They no longer have an air force to fly and protect themselves," the official said on condition of anonymity.

"Essentially the Russians now have complete air superiority over Ukraine."

- Airfield battle -

Neither official offered estimates of how many Russian troops and armored vehicles had crossed the border and entered Ukraine.

They both said Ukraine forces were putting up a fight.

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

A principal Russian target was the airfield at Gostomel, less than 20 kilometers (12 miles) from the center of Kyiv.

Russian paratroopers appeared to seize of the facility, but Ukraine officials later claimed they had driven the Russians out.

The airport, military analysts say, could become a key staging point for the Russians, to fly in more troops in preparation for a siege on Kyiv.

"If Moscow can hold onto it, and continue to achieve air superiority (the latter is very likely), it will most certainly use the airport as an entry point to attack Kyiv," said Michael Horowitz, a security analyst at consultant Le Beck International, in a tweet.

- 7,000 US troops to Germany -

Meanwhile, the Pentagon announced the United States was sending 7,000 more troops to Europe.

"They will deploy to Germany to reassure NATO Allies, deter Russian aggression and be prepared to support a range of requirements in the region," a Pentagon official said, adding they are expected to depart "in the coming days."

The 7,000 brings the number of soldiers sent from the United States to Europe to 12,000.

But President Joe Biden reiterated Thursday that while Washington will stand by its NATO allies militarily, no American forces will be sent to fight Russians in Ukraine, which is not a member of the Atlantic alliance.