Ryanair has said that all of its flights to and from Ukraine have been suspended for at least the next 14 days.
The airline, which is one of the largest foreign airlines in Ukraine, added that all flights to and from Ukraine have also been removed from sale for at least the next four weeks.
Ryanair said it remains committed to its services in Ukraine, adding that it looks forward to restoring them as soon as it is safe to do so.
Meanwhile, Ukraine closed its airspace to civilian flights today after Russia launched a land, sea and air invasion of its neighbour, while Europe's aviation regulator warned about hazards to flying in bordering regions.
Moldova, southwest of Ukraine, also closed its airspace, while Belarus to the north said civilian flights could no longer fly over part of its territory after Russian President Vladimir Putin authorised the military operation.
Ukraine State Air Traffic Services Enterprise said on its website that the country's airspace was closed to civilian flights today. Air traffic services were also suspended.
The European Union Aviation Safety Agency (EASA) said airspace in Russia and Belarus within 100 nautical miles of their borders with Ukraine could also pose safety risks.
"In particular, there is a risk of both intentional targeting and misidentification of civil aircraft," the agency said in a conflict zone bulletin.
"The presence and possible use of a wide range of ground and airborne warfare systems poses a high risk for civil flights operating at all altitudes and flight levels," it added.
The aviation industry has taken heightened notice of the risks conflicts pose to civil aviation since Malaysia Airlines flight MH17 was shot down over eastern Ukraine in 2014, when fighting in the region flared.
EASA said Russia's defence ministry had sent Ukraine an urgent message warning of a high risk to flight safety, due to the use of weapons and military equipment and asked Ukraine's air traffic control to stop flights.
Websites before the escalation had shown intelligence-gathering flights over or near Ukraine as the West showcased support by transmitting detectable signals in recent weeks,.
But they showed empty space this morning as aircraft left and Ukraine was declared a conflict zone.
Early morning airline traffic skirted the whole country in crowded corridors to the north and west.
Russia said today it had suspended domestic flights to and from several airports near its border with Ukraine, including Rostov-on-Don, Krasnodar and Stavropol, until March 2.
Russia closed some airspace in the Rostov sector "in order to provide safety" for civil flights, according to a notice to air crew, known as a NOTAM.
Before Ukraine's announcement, Britain, Canada, France, Italy and the US told their airlines to avoid certain airspace above eastern Ukraine and Crimea but stopped short of a total ban.
London-listed budget carrier Wizz Air said it was halting operations out of Ukraine in an announcement released after Ukraine shut its airspace.
Germany's Lufthansa and KLM suspended flights days earlier.
