Shipping companies are not rushing to export millions of tons of grain trapped in Ukraine, despite a breakthrough deal to provide safe corridors through the Black Sea. That is because explosive mines are drifting in the waters, ship owners are assessing the risks and many still have questions over how the deal will unfold.

The complexities of the agreement have set off a slow, cautious start, but it's only good for 120 days – and the clock began ticking last week.

The goal over the next four months is to get some 20 million tons of grain out of three Ukrainian sea ports blocked since Russia's Feb. 24 invasion. That provides time for about four to five large bulk carriers per day to transport grain from the ports to millions of impoverished people worldwide facing hunger.

It also provides ample time for things to go awry. Only hours after the signing Friday, Russian missiles struck Ukraine's port of Odesa – one of those included in the agreement.

Another key element of the deal offers assurances that shipping and insurers carrying Russian grain and fertilizer will not get caught in the wider net of Western sanctions. But the agreement brokered by Turkey and the UN is running up against the reality of how difficult and risky the pact will be to carry out.

"We have to work very hard to now understand the detail of how this is going to work practically," said Guy Platten, secretary-general of the International Chamber of Shipping, representing national shipowners associations that account for about 80% of the world's merchant fleet.

"Can we make sure and guarantee the safety of the crews? What's going to happen with the mines and the minefields, as well? So lots of uncertainty and unknowns at the moment," he said.

Getting wheat and other food out is critical to farmers in Ukraine, who are running out of storage capacity amid a new harvest. Those grains are vital to millions of people in Africa, parts of the Middle East and South Asia, who are already facing food shortages and, in some cases, famine.

Ukraine and Russia are key global suppliers of wheat, barley, corn and sunflower oil, with fighting in the Black Sea region, known as the "breadbasket of the world," pushing up food prices, threatening political stability in developing nations and leading countries to ban some food exports, worsening the crisis.

UN humanitarian chief Martin Griffiths says work at the newly opened Joint Coordination Center in Istanbul overseeing the export deal is "non-stop with the aim of seeing the shipments heading out of Ukrainian ports quickly, safely and effectively."

He said Frederick Kenney, Jr., director of legal and external affairs at the International Maritime Organization and a retired U.S. Coast Guard rear Admiral and judge advocate, is leading the UN's efforts to get the grain deal up and running.

The deal stipulates that Russia and Ukraine will provide "maximum assurances" for ships that brave the journey through the Black Sea to the Ukrainian ports of Odesa, Chernomorsk and Yuzhny.

"The primary risk that's faced is obviously going to be mines," said Munro Anderson, head of intelligence and a founding partner at Dryad. The maritime security advisory company is working with insurers and brokers to assess the risks that ships could face along the route as sea mines laid by Ukraine to deter Russia are drifting.

Turkey's defence minister on Wednesday said demining the waters was not immediately required but that could change.

Ukrainian officials have expressed hope that exports could resume from one port within days, but they also have said it could take two weeks for all three to become operational again. Experts in Ukraine are working on determining safe routes for ships.

Shipowners, charterers and insurance firms, meanwhile, are trying to understand how the deal will play out.

"I think it's going to come (down) to the position of the marine insurers that provide war risk and how much they are going to be adding in additional charges for vessels to go into that area," said Michelle Wiese Bockmann, shipping and commodities analyst at Lloyd's List, a global shipping news publication.

Bockmann said vessels carrying this kind of load typically have between 20 to 25 seafarers on board.

"You can't risk those lives without something concrete and acceptable to the shipowners and to their charterers to move grain," she said.

Oleksiy Melnyk, an analyst with the Kyiv-based Razumkov Center think tank, said safety issues are largely unresolved because Russian rockets can hit warehouses storing grain and ports.

"Shipowners and insurance companies are scared, they haven't received any reliable security guarantees," Melnyk said. "We're seeing just words and promises, which are worth little at a time of war."

Marine insurers reached by AP declined to comment on whether they would provide coverage for these ships.

The war has wreaked havoc on global trade, stranding over 100 ships in Ukraine's many ports.

At the three ports in the export agreement, 13 bulk carriers and cargo ships have been stuck at Chornomorsk, six in Odesa and three at Yuzhny, data from Lloyd's List Intelligence shows. Some of those ships might still have crews aboard that could be mobilized to start exporting grains.

Ukrainian traders have been able to send some grain through the Danube River, which helped buoy exports to about 1.5 million tons in May and up to 2 million tons in June, though that is still less than half the monthly grain shipments of 4 to 5 million tons before the war, according to Svetlana Malysh, a Black Sea agriculture markets analyst with Refinitiv.

Over the 2021-2022 marketing year, Russia exported about 30 million tons of wheat, according to Refinitiv trade flows. That is the lowest level since 2017, in part because of the chilling effect of sanctions. Russian fertilizer exports also dropped 25% in the first quarter of the year compared with the same period last year, Malysh said.

For ships heading to Ukraine's three ports, smaller Ukrainian pilot boats will guide the vessels through approved corridors. The entire operation will be overseen by a Joint Coordination Center in Istanbul staffed by officials from Ukraine, Russia, Turkey and the United Nations.

Once ships reach port, they will be loaded with tens of thousands of tons of grains before departing back to the Bosphorus Strait, where they will be boarded to inspect them for weapons. There will likely be inspections for ships embarking to Ukraine as well.

Because the process is so complex and slow-moving, it's unlikely to have a significant impact on the price of grain worldwide.

"The balance of power on this agreement still sits with Russia," said Anderson, Dryad's head of intelligence. Any Ukrainian ports outside the agreement face increased risk of attack, he said.

"I think what Russia wants ... is to be seen as the state that controls the narrative within the Black Sea," Anderson said.

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Associated Press writers Edith M. Lederer at the United Nations and Susie Blann in Kyiv, Ukraine, contributed to this report.