Ukrainian forces have withdrawn from the airport of the eastern city of Luhansk after all-night clashes with pro-Russian rebels, the military says. Security officials said the Ukrainian troops had come under attack from a column of Russian tanks. Russian Foreign Minister Sergei Lavrov has urged negotiators meeting in the Belarus capital, Minsk, to make an "immediate ceasefire" their priority. Some 2,600 people have died in eastern Ukraine since fighting began in April. The conflict broke out after Russia's annexation of Ukraine's southern Crimea peninsula in March.