Ukraine's prime minister says his government has decided to close the country's airspace to all Russian planes.Ukraine last month banned all Russian airlines from flying into Ukraine but Russian planes have been allowed to fly over its territory.The decision to shut the airspace was announced by Arseny Yatsenyuk at a televised government session on Wednesday. It comes as tensions simmer between Russia and Ukraine following Russia's annexation of Crimea last year and a separatist war in eastern Ukraine which has killed more than 8,000 people and left parts of the industrial heartland in ruins.The Russian gas monopoly Gazprom earlier on Wednesday announced that it was cutting off gas supplies to Ukraine because the country had not paid in advance for supplies.