The four-way meeting in Istanbul comes with Russia’s invasion of Ukraine showing no sign of abating and the threat of food shortages spreading across the poorest parts of the world.

Ukraine is a vital exporter of wheat and grains such as barley and maize. It has also supplied nearly half of all the sunflower oil traded on global markets.

But exports across the Black Sea have been blocked by Russian warships and mines Kyiv has laid to avert a feared amphibious assault.

The negotiations are being complicated by growing suspicions that Russia is trying to export grain it has stolen from Ukrainian farmers in regions under its control.

US space agency data released last week showed 22 percent of Ukraine’s farmland falling under Russian control since the February 24 invasion.

UN Secretary-General Antonio Guterres tried on Tuesday to play down expectations from the Istanbul talks.

“We are working hard indeed, but there is still a way to go,” the UN chief told reporters.

The meeting will involve military delegations from the three countries and diplomats from the UN.

Turkish Defence Minister Hulusi Akar said they would focus “on the safe shipment to international markets of grain waiting in Ukrainian ports”.

– Safe corridors –

NATO member Turkey has been using its good relations with both the Kremlin and the Western-backed leaders in Kyiv to try and broker an agreement on a safe way to deliver the grain.

Turkey says it has 20 merchant ships waiting in the region that could be quickly loaded and sent to world markets.

A plan by the UN proposes the ships follow safe “corridors” that run between the known location of mines.

Experts say de-mining the Black Sea is a complex operation that could take months — too long to address the growing global food crisis.

Ukraine estimates that up to 25 million tonnes of grain are currently blocked in its ports.

A report by the International Rescue Committee international relief group warned last month that 47 million people faced the threat of “acute hunger” this year.

The talks have been gaining momentum since Ukraine’s recapture this month of the tiny but strategic Snake Island from the Russians.

The uninhabited rock sits near routes used to export the grain.

Its return has already enabled Ukraine to start the first shipments along the nearby Danube River running to Romania.

– Erdogan-Putin meeting –

The talks in Istanbul precede a meeting in Tehran next Tuesday between Turkish President Recep Tayyip Erdogan and his Russian counterpart Vladimir Putin.

The two leaders’ first meeting since Russia’s invasion will be held on the sidelines of a three-way summit on Syria hosted by Iranian President Ebrahim Raisi.

The war in Ukraine has contributed to Turkey’s mounting economic problems and further complicated Erdogan’s path to a third decade in power in elections due within the next year.

Erdogan has been offering to meet Putin for months — only to be rebuffed.

Both Ukraine and Russia have laid out firm demands entering Wednesday’s talks.

A Russian foreign ministry spokesman said Moscow wanted to be able to “control and search the ships to avoid the contraband of weapons, and Kyiv’s commitment not to stage provocations”.

Ukraine said it sought to ensure any solution did not threaten “the security of our southern regions” along the Black Sea.

Kyiv has also asked that its merchant vessels be accompanied by warships from a friendly country such as Turkey.