Ceasefires need trust and for Ukraine-Russia's grain exports deal that's in scarce supply

If you're an optimist this agreement is an extraordinary piece of diplomacy which should help ease hunger in some of the most impoverished places in the world.

But if you are a pessimist, the ink on this document may not be worth much in practice - Ukraine and Russia are still locked in an ugly fratricidal war where enmity and suspicion of motives know no bounds.

For the moment the glass half full brigade probably has it.

The agreement known as the Black Sea Initiative is really a very focused ceasefire at sea.

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For instance, Ukraine's main port city of Odessa will not be protected from hostilities but the parts of it needed for the export of grain will be.

Operationally exports will be overseen by a Joint Control Centre - manned by the four partners: the UN, Turkey, Russia and Ukraine.

Ukraine will guarantee and pilot ships through safe passages. The Black Sea has been heavily mined to deter a Russian amphibious assault and Russia will guarantee not to open fire as ships make their way to the Bosphorus and Turkish waters.

But like all ceasefires the real commodity in all of this will be trust and that - diplomacy, or no diplomacy - is in scarce supply.

Russia stands accused of weaponising food in this conflict - as a tactic by exacerbating the migrant crisis from the global south it can dictate, to a certain extent, Europe's politics.

Large numbers of hungry Africans turning up on the continent - the argument goes - will weaken resolve amongst the electorate to keep supporting Ukraine's war and the supply of Western guns that help Kyiv fight.

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Food as a weapon also strongly persuades "partners", particularly in Africa to support, or at least not be critical of its invasion of a sovereign nation.

The Kremlin, of course, denies all this and for its part blames draconian Western sanctions for the food crisis.

Either way it's easy to see how this fragile agreement could break down.

And if it does, it will not only be Ukrainian farmers who'll pay the price, but millions of people who are already struggling in global cost of living crisis to eke out a basic existence.