Russian company Gazprom said Wednesday it would stop natural-gas supplies to Ukraine until Kiev pays for more energy. The company said it would not send more gas until Kiev pays in advance. Chief Executive Alexei Miller warned that halting shipments would threaten the secure transit of gas to Europe through Ukraine. An estimate 15 percent of the gas used in Europe travels through Ukraine. Previous disputes between Moscow and Kiev have led to stoppages and the latest halt comes less than two months after Russia resumed gas shipments as part of an EU-brokered deal to secure supplies until March. Ukrainian Prime Minister Arseny Yatsenyuk, meanwhile, said Wednesday that the country is closing its airspace to all Russian planes.