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The US government expects a large-scale Russian invasion of Ukraine aimed at "decapitating the government" in Kiev, a defence official said on Thursday.
The official added that early Russian attacks, which have killed up to 40 people and have hit areas close to the Ukrainian capital, were only the beginning.
"What we are seeing are the initial phases of a large-scale invasion," a senior Pentagon official told reporters.
"How long, how far, we don't know."
US officials say the Russian attack so far has involved a barrage of more than 100 missiles and about 75 aircraft and bombers as Moscow seeks regime change in Kiev.
“They have every intention of basically decapitating the government and installing their own means of governance,” the official said.
Washington is tracking three front lines in the Russian invasion, from Belarus, Crimea and the north-eastern border with Ukraine.
The Pentagon has reported ground resistance from Ukraine but did not comment on reports of casualties.
On Thursday, President Joe Biden said he is authorising the deployment of additional US forces to Germany.
"The United States will defend every inch of Nato territory with the full force of American power,” Mr Biden said.
"This is a dangerous moment for all of Europe, for the freedom around the world .. [and] on the very principles that uphold with global peace.”
A senior US defence official said the additional deployment consists of about 7,000 troops.
"At the direction of the president, Secretary of Defence Austin has ordered the deployment to Europe of approximately 7,000 additional servicemembers," the official said.
"This would comprise an armoured brigade combat team with associated capabilities and enablers."
US defence efforts so far have been primarily focused on bringing supplies to Ukraine and helping Poland accommodate refugees.
A large number of the 5,500 US troops who arrived in Poland this month have moved closer to the border and are working with Polish forces to set up three processing centres to help the tens of thousands of people fleeing Ukraine, The New York Times reported.
Mr Austin vowed to send more anti-tank missiles to Ukraine this week.
He told Ukraine’s Foreign Minister Dmytro Kuleba that US support for Ukraine's self-defence, sovereignty and territorial integrity is “unwavering".
Since the 2014 Russian annexation of Crimea, the US has provided Ukraine with more $2.7 billion in security assistance, including more than $650 million last year.
