By Tuvan Gumrukcu ISTANBUL (Reuters) – The United Nations expects a deal to resume Ukrainian grain exports through the Black Sea to be fully operational in a few weeks and restore shipments to pre-war levels of 5 million tonnes a month, two senior U.N. officials said on Friday. Ukraine, Russia, Turkey and the United Nations signed the deal in Istanbul in what Turkey has said will mark the first step toward easing an international food crisis exacerbated by Moscow’s invasion of its neighbour. Russia and Ukraine are among the world’s top food exporters and Moscow’s Feb. 24 invasion has blockaded…