Ukraine said on Thursday it had closed its airspace to civilian flights because of a "high risk" to safety, and Europe's aviation regulator also warned about the hazards of flying in bordering areas of Russia and Belarus because of military activities.
Russian President Vladimir Putin authorized a military operation in eastern Ukraine on Thursday in what appeared to be the start of war in Europe.
Ukraine State Air Traffic Services Enterprise said on its website that the country's airspace was closed to civilian flights starting at 0045 GMT on Thursday and that air traffic services had been suspended.
The European Union Aviation Safety Agency (EASA) said airspace in Russia and Belarus within 100 nautical miles (185 kilometers) of their borders with Ukraine could also pose safety risks to airlines.
"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."
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.
EASA said Russia's Ministry of Defense had sent an urgent message to Ukraine warning of a high risk to flight safety due to the use of weapons and military equipment from 0045 GMT on Thursday and asked Ukraine's air traffic control to stop flights.
Flight tracking websites showed early morning traffic skirting the whole country in crowded corridors to the north and west.
An El Al flight from Tel Aviv to Toronto made a sudden U-turn out of Ukraine's airspace around the time the airspace was closed, according to flight-tracking website FlightRadar24.
A LOT Polish Airlines flight from Warsaw to Kyiv also turned back to Warsaw around the same time.
Hours before then, Safe Airspace, which was set up to provide safety and conflict zone information after the downing of MH17, said it had increased its risk level over all of Ukraine to "do not fly."
It also warned of the potential for a cyberattack on Ukraine's air traffic control.
Russia has also closed some airspace in the Rostov sector to the east of its border with Ukraine "in order to provide safety" for civil aviation flights, according to a notice to airmen.
Before Ukraine advised of the airspace restrictions, the United States, Italy, Canada, France and Britain had told their airlines to avoid certain airspace above eastern Ukraine and Crimea but stopped short of a total ban.
Germany's Lufthansa LHAG.DE halted flights to Ukraine from Monday, joining KLM which already suspended flights.
Two Ukrainian airlines last week disclosed problems in securing insurance for some of their flights while foreign carriers began avoiding the country's airspace as Russia massed a huge military force on its border.