Airlines should stop flying over any part of Ukraine because of the risk of an unintended shootdown or a cyber attack targeting air traffic control amid tensions with Russia, a conflict zone monitor said on Wednesday.

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."

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.

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Separatists in Ukraine asked on Wednesday for Russian help to repel "aggression" and Kyiv announced compulsory military service and a state of emergency as the West slapped more sanctions on Moscow in a bid to stop an all-out invasion.

The United States, Italy, Canada, France and Britain have advised their airlines to avoid certain airspace above eastern Ukraine and Crimea but so far have stopped short of a total ban. Safeairspace.net said it expected updated guidance could be issued soon.

The US Federal Aviation Administration said in its latest guidance on Feb. 9 that a cross-border conflict between Ukraine and Russia could pose a direct or indirect threat to civil aviation.

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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.

Several airlines were still flying over Ukraine on Thursday, according to a Reuters check of flight tracking website FlightRadar24.