The first wartime wheat from Ukraine should ship next week under a landmark deal also signed by Russia aimed at tackling the global food crisis, a top UN official said on Wednesday.

The first 12 ships to leave the three Black Sea ports designated by the agreement were carrying 370,000 tonnes of corn and foodstuffs, according to Frederick Kenney, interim UN coordinator at the joint centre in Istanbul overseeing the deal.

But that should change once the ships docked in Ukraine when Russian invaded its neighbour in February leave their ports and new ones come in to pick up wheat that has accumulated with this year's harvest, Kenney told reporters.

"We are dealing with three ports that were essentially frozen in time," Kenney said.

"The silos were full of corn and the ships that were there have been loaded with corn," he said.

"It's imperative to get those ships out to get new ships in .... that can deal with the food crisis."

The agreement, signed by the warring parties and UN and Turkish officials last month in Istanbul, was hailed as a major opportunity to tackle the global food crisis caused by the war.

But Ukraine needed to first deal with the corn -- much of it used as animal feed -- that was contractually agreed with global clients at the time of Russia's invasion.

"We're actually transitioning to wheat," Kenney said.

"We have cleared the first ship inbound" to Ukraine through the Bosphorus Strait, he said. "That should occur some time next week."

The deal was dealt an early setback when the first ship to leave Ukraine -- the Sierra Leone-flagged vessel Razoni -- failed to reach its destination in Lebanon because of a contractual dispute.

Ukrainian officials said the shipment's five-month delay prompted the Lebanese buyer to cancel the deal once the ship was already at sea.

The Razoni docked in Turkey's Mediterranean Sea port of Mersin on Wednesday following a report that its maize has found a new buyer.

The "cargo sold.... It will (unload) at Mersin," shipping agent Ahmed al-Fares of the Ashram Maritime Agency told the Middle East Eye news site.

Kenney stressed that his Istanbul centre did not get involved in contractual disputes, focusing on its mission of safely navigating ships through a designated corridor cutting through mine-strewn Black Sea waters.

The ships are inspected coming into and out of the Bosphorus.

"We're seeing steady progress in the number of ships coming in and out," he said. "We're off to a good start."