A number of airlines have grounded flights as Ukraine's president has declared martial law and the country's foreign minister says a "full-scale invasion" has begun.
Explosions have been heard and seen in the Ukrainian capital of Kyiv and other major cities following Vladimir Putin's announcement that a military operation was underway.
The sound of distant blasts were picked up in a live broadcast from US outlet CNN, causing reporter Matthew Chance to put on a flak jacket.
READ MORE: Live updates as Putin launches Russian invasion of Ukraine and explosions rock cities
The Russian president announced the action during a televised address early on Thursday morning, saying the move was a response to threats from Ukraine.
Prime Minister Boris Johnson said Russian President Vladimir Putin has "chosen a path of bloodshed and destruction" with his attack.
He said that the UK and its allies would respond "decisively".
Ryanair has suspended its Ukraine flights for at least the next fortnight.
The airline said: "Due to the closure of Ukrainian airspace overnight and the apparent invasion by Russian forces, all Ryanair flights to/from Ukraine have been suspended for at least the next 14 days.
"All affected passengers will receive email notices later this morning and all flights to/from Ukraine have been removed from sale for at least the next four weeks until further information becomes available from EU safety agencies.
"Ryanair remains committed to our services to/from Ukraine and we look forward to restoring flight services there as soon as it is safe to do so.
"We sincerely regret and apologise for these unprecedented disruptions and any inconvenience that they will inevitably cause to our Ukrainian customers."
Wizz Air has also, today, said it will suspend all flight operations in Ukraine.
The carrier, which operates flights between London Luton and the eastern European country, said: "Due to the current events in Ukraine and the airspace closure, Wizz Air regrets to inform our customers that the airline must temporarily suspend all flight operations in the country.
"Passengers booked on arriving or departing Ukrainian flights can find more information on wizzair.com, or alternatively via the airline's call centre: 00380 893 202 532.
"The safety and security of our passengers and crew remains our number one priority and we hope normality will return to Ukraine soon."
The news comes after Air France, Lufthansa and KLM all cancelled or suspended flights into Kiev earlier this week.
Air France said: "In light of the local situation and the closure of Ukrainian airspace, Air France extends the suspension of its service to Kiev (Ukraine) until further notice. The customers concerned have been informed individually.
"Air France reminds that the safety and security of its flights, its customers and its crews, is an absolute imperative."
Dutch airline KLM made the decision to stop all flights to and from the country on February 12.
The decision, it said, "follows the adjusted travel advice to code red and an extensive safety analysis. It is not yet clear when KLM will fly to Kiev again."
Scandinavian company SAS has also suspended its weekly flights.
All aircraft flying to or from UK airports are being ordered to avoid Ukraine airspace after the crisis in the country intensified.
Transport Secretary Grant Shapps said he made the decision "following the horrific events overnight" as Russia launched a major military assault.
The Cabinet minister tweeted: "I've instructed @UK-CAA (the Civil Aviation Authority) to ensure airlines avoid Ukraine airspace to keep passengers and crew safe.
"We continue to stand with the people of Ukraine and work with our international partners to respond to this act of aggression."
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