Ukraine's president has told airlines that Russian airspace will be "effectively closing" as his country escalates its drone strikes. "We want to warn every airline that uses Russian airspace, every insurer, everyone who still uses key Russian airports: the Russian sky is becoming completely dangerous," Volodymyr Zelensky says.Most Western commercial airlines do not fly over Russia as a consequence of the ongoing Russian invasion of Ukraine, but carriers in countries such as China do.Russian President Vladimir Putin responded by saying that Zelensky's comments amounted to a "declaration of state terrorism", while adding that Russia would continue its own intensifying attacks on Ukraine.Explaining the elevated airspace risk, Zelensky said: "Ukraine does not threaten civilian aviation as such - not a single civilian aircraft. "There will simply be drones in Russia's skies on a scale that has to be taken into account," he said in his nightly video address on Tuesday.Zelensky said that "the war started by Russia is closing its skies".He directed the warning to airlines continuing to operate flights to Russian airports, especially in major cities like St Petersburg and Moscow.Ukraine has targeted energy facilities deep inside Russian territory in addition to large retailers. Aviation risk advisory firm Osprey Flight Solutions said in an alert to clients that while the majority of Ukraine's drone and missile strikes were likely to occur within 300km (186 miles) of the border, daily attacks deeper inside Russia were almost certain to continue this year."There is also a realistic possibility that the frequency of Ukrainian strikes will continue to increase in the short term deep inside Russian territory," the alert added. "It is assessed that miscalculation and/or misidentification is likely in areas related to the Ukraine conflict."Speaking to reporters at the Shanghai Co-operation Organisation summit in Kyrgyzstan, Putin said: "They are asking us to resume negotiations... but you don't negotiate with terrorists."His comments come as Russia's attacks on Ukraine continue, with heavy strikes killing 12 people on Monday. The military has said it is preparing to carry out "massive" strikes on Ukrainian power stations.Putin also acknowledged that Ukrainian strikes had damaged infrastructure inside Russia, while insisting that "only 10%" of oil refineries were left to repair. Chinese, Turkish and Gulf state airlines are among those using Russian airspace, with savings in time and fuel on routes between Asia and Europe. Other carriers have largely avoided the eastern European region since the beginning of Russia's full-scale invasion in February, 2022.Ukraine has closed its airspace to civilian aircraft since the beginning of the war.Last year, the UN aviation agency ruled that Russia was responsible for the downing of Malaysia Airlines Flight MH17 over eastern Ukraine in July 2014.All 298 people on board the passenger plane were killed when it was shot down by a Russian-made missile. The Kremlin has always denied any responsibility for the air disaster.In 2024, Putin apologised to the president of neighbouring Azerbaijan over the downing of a commercial airliner in Russian airspace, in which 38 people were killed.He said the "tragic incident" had occurred when Russian air defence systems were repelling Ukrainian drones.