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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.
A Ukrainian soldier walks past the shell of a burnt military vehicle, on a street in Kiev. AP
A screengrab from CCTV footage shows a missile hitting a residential building in Kiev. Reuters
A man inspects a broken window, after shelling carried out by Russian troops in the Ukrainian capital. Reuters
Ukrainian soldiers take positions outside a military centre as two cars burn on a street in Kiev. AP
Medical specialists transport a woman, who was wounded in the shelling of an apartment building, to an ambulance, as her husband looks on. Reuters
A woman outside her apartment building after it was damaged by a rocket, in Kiev. AP
A Ukrainian police officer detains a driver on a street in Kiev. AP
A soldier's helmet with a bullet hole on a street in the Ukrainian capital. AP
Ukrainian soldiers walk by a damaged vehicle, at the site of fighting with Russian troops, in Kiev. Reuters
Ukrainian policemen move parrots to safety after a high-rise apartment block was damaged by shelling in Kiev. EPA
A screengrab from a video shows rescue workers carrying a person at the site of a damaged multi-storey residential building, south-west of the Ukrainian capital. Reuters
A building damaged by shelling carried out by Russian troops in Kiev. EPA
Helga Tarasova comforts her daughter Kira Shapovalova as they wait in an underground shelter during a bombing alert in Kiev. AFP
A man walks past a building damaged by a rocket attack in the Ukrainian capital. AP
Emergency services workers near an apartment building damaged by shelling in Kiev. Reuters
Ukrainian soldiers collect unexploded shells in the capital. AFP
An apartment building damaged by shelling in Kiev. Reuters
Debris of a burnt military truck on a street in Kiev. AP
Natali Sevriukova stands next to her home after a rocket attack in Kiev, Ukraine. AP
People on foot and in cars move to cross from Ukraine to Poland at the Korczowa-Krakovets border crossing following the Russian invasion of Ukraine. AFP
Servicemen from the Ukrainian National Guard in central Kiev. Reuters
Ukrainian soldiers in downtown Kiev after Russian troops reached the outskirts of the capital. AP Photo
Police officers detain demonstrators in St. Petersburg, Russia. Hundreds of people gathered in Moscow and St. Petersburg on Thursday, protesting against Russia's attack on Ukraine. Similar protests took place in other Russian cities, and activists were also arrested. AP Photo
Hungarian troops in Vasarosnameny, close to the border with Ukraine, prepare to help refugees. AP Photo
Wrecked Russian army rocket launchers in Kharkiv, Ukraine. Reuters
The aftermath of a rocket attack in Kiev. AP Photo
A US Air Force jet refuels over Poland. Reuters
Devastation after shelling in Ukraine's Luhansk enclave. Reuters
The Royal Welsh Battlegroup from the UK makes its way to Estonia. Reuters
Firefighters tackle a blaze in a building in Kiev. AFP
The wreckage of an unidentified aircraft in a residential area in Kiev. Reuters
Smoke rises near the Ukrainian Defence Ministry in Kiev. Reuters
Russian military helicopters fly over the Ukrainian capital. AP
An anxious wait to board a train to leave Kiev as the attack loomed. AP Photo
Ukrainian servicemen on tanks in the Donetsk region, eastern Ukraine. AP Photo
Ukraine troops said this destroyed armoured vehicle outside Kharkiv belonged to the Russian army. Reuters
Rescuers at the crash site after a Ukraine armed forces Antonov aircraft was shot down in Kiev, according to the Ukrainian authorities. Reuters
Ukrainian tanks that aim to repel an attack by the Russian military in the Luhansk enclave of Ukraine. AFP
A Ukrainian State Border Guard Service base is ablaze after heavy shelling near Kiev. Reuters
Damaged radar at a Ukrainian military plant outside Mariupol. Officials in Kiev say Russian troops have rolled into the country from the north, east and south. AP Photo
Firefighters attend a blaze after bombing in the eastern Ukrainian town of Chuguiv, as Russian armed forces appeared to be bombarding Ukraine from several directions. AFP
A traffic jam in Kiev, with many Ukrainians leaving their capital after Russian troops entered. EPA
The Border Service of Ukraine released CCTV footage purporting to show Russian military vehicles passing a checkpoint to drive into Crimea. AP
A man picks up fragments of military materiel on the street after an apparent Russian strike in Kharkiv, Ukraine. AP
Workers load the debris of a rocket on to a lorry in the aftermath of Russian shelling in Kiev. AP
Emergency staff treat an injured man after bombing in Chuguiv. AFP
An American woman in distress after crossing the border to flee the violence in Ukraine, in Medyka, Poland. Reuters
People take shelter in a Kiev subway station, after Russian President Vladimir Putin authorised a military operation in eastern Ukraine. Reuters
People in a rush near Kiev-Pasazhyrskyi railway station in the Ukrainian capital. AFP
A Flightradar24.com screengrab shows a dearth of civilian aircraft in Ukrainian airspace after the Russian invasion. AP
The aftermath of bombing in Chuguiv. AFP
A Ukrainian rocket launcher vehicle prepares for action. Reuters
Cars are driven across a field to leave the city of Kharkiv. Reuters
People chant, carry placards and wave Ukrainian flags as they gather in Sydney, to demonstrate against Russian attacks on its neighbour. AP
People with many of their belongings packed take shelter in a Metro station in Kiev. AFP
Emergency services at the scene of an explosion at a military unit building in Kiev. EPA
A Kiev resident sweeps up debris after Russian shelling. AP
The first 40 soldiers of the 173rd US Army Airborne Brigade step off a military plane in Latvia. EPA
More damage from shelling in Kiev. AP
People wait at a bus station to travel to western parts of Ukraine, after violence in the east. Reuters
A woman carries her cats in a quest for a safe haven. AFP
Ukrainian President Volodymyr Zelenskyy holds an urgent government meeting in Kiev to discuss the next step. AFP
European Commission President Ursula von der Leyen and foreign policy chief Josep Borrell give a joint statement in Brussels, updating the press about the situation in Ukraine. EPA
People survey the damage after bombing in Chuguiv. AFP
A building on fire in Chuguiv. AFP
Updated: February 24, 2022, 4:40 PM