Russian President Vladimir Putin authorised a military operation in eastern Ukraine on Thursday in what appeared to be the start of war in Europe.
"The provision of air traffic services to civilian users of the airspace of Ukraine is suspended," Ukrainian State Air Traffic Services Enterprise said on its website on Thursday.
"We will additionally inform about changes in the use of Ukraine's airspace," the agency added, without providing further details.
Eurocontrol, which coordinates air traffic in Europe, said that Ukraine's airspace was not available because of military restrictions.
An El Al flight from Tel Aviv to Toronto made a sudden U-turn out of Ukraine's airspace around the time a notice to airmen was issued early on Thursday deeming the area restricted, according to flight tracking website FlightRadar24.
A LOT Polish Airlines flight from Warsaw to Kyiv also turned back to Warsaw around the same time.
The turnarounds came after Safe Airspace, which was set up to provide safety and conflict zone information for airlines after Malaysia Airlines flight MH17 was shot down over eastern Ukraine in 2014, said it had increased its risk level to "do not fly".
"Regardless of the actual movements of Russian forces into Ukraine, the level of tension and uncertainty in Ukraine is now extreme," Safe Airspace said on its website. "This itself gives rise to significant risk to civil aviation."
It also warned of the potential for a cyberattack on Ukraine's air traffic control.
Russia has closed some airspace in the Rostov flight information region 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.
The US Federal Aviation Administration said in its latest guidance on February 9 that a cross-border conflict between Ukraine and Russia could pose a direct or indirect threat to civil aviation.
Germany's Lufthansa 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.
