Turkey says a deal has been reached with Russia to allow Ukraine to resume exports of grain through the Black Sea. It is to be signed on Friday in Istanbul by Ukraine, Russia, Turkey and UN Secretary General Antonio Guterres. The world shortage of Ukrainian grain since Russia's 24 February invasion has left millions at risk of hunger. The invasion sent food prices soaring, so the deal to unblock Ukraine's ports is crucial. Some 20 million tonnes of grain is stuck in silos in Odesa.