Turkey, the United Nations, Russia and Ukraine signed the groundbreaking grain corridor deal in Istanbul on Friday.

Russia and Ukraine account for nearly a third of global wheat supplies, while Russia also exports fertilizer and Ukraine corn and sunflower oil. But Ukrainian grain shipments from its Black Sea ports have stalled since Russia invaded, with some 20 million tons of grain stuck.

The U.N. has appealed to both countries, as well as maritime neighbor Turkey, to create a sea corridor for Ukrainian grains to be exported from the Black Sea. Turkey has been coordinating with Russia and Ukraine to agree to a plan that would restart grain exports from Ukrainian ports.

Ankara has held talks with Moscow and the U.N., but says all sides need to meet for a final agreement.

Ankara has offered to host an "observation mechanism" to be formed to monitor the implementation of the sea corridor plan in Istanbul.