The first shipment of Ukrainian grain since the Russian invasion in February left the port of Odessa on Monday under a landmark deal to lift Moscow's naval blockade in the Black Sea.

United Nations chief Antonio Guterres, who brokered the plan with Turkey, welcomed the announcement. Kyiv said it would bring "relief for the world" if Moscow held up its side of the accord.

The five-month halt of deliveries from war-torn Ukraine -- one of the world's biggest grain exporters -- has contributed to soaring food prices, hitting the world's poorest nations especially hard.

Officials said the Razoni cargo ship, registered in Sierra Leone, was making its way through a specially cleared corridor in the mine-infested waters of the Black Sea with 26,000 tonnes of maize on board.

"It is expected in Istanbul on August 2. It will then continue its journey after it has been inspected in Istanbul," the Turkish foreign minister said in a statement.

Other convoys would follow, respecting the maritime corridor and the agreed formalities, the statement said.

Last month, Ukraine and Russia signed the breakthrough pact -- the first signficant accord involving the warring sides since the invasion -- with Turkey and the United Nations aimed at getting millions of tonnes of trapped Ukrainian grain to world markets.

But Russian strikes on the Odessa port the day after the deal was signed sparked outrage from Ukraine's allies and cast doubt over the accord.

Guterres said he hoped the Razoni would be the first of many commercial ships to depart from Ukraine and that "this will bring much-needed stability and relief to global food security, especially in the most fragile humanitarian contexts".

"Ensuring that existing grain and foodstuffs can move to global markets is a humanitarian imperative".

Ships 'waiting to leave

He also said the World Food Programme was planning to "purchase, load and ship an initial 30,000 metric tons of wheat out of Ukraine on a UN-chartered vessel," with more details in the coming days.

EU spokesman Peter Stano said the departure Monday was a good "first step" but that Brussels still expects the "implementation of the whole deal and resumption of Ukrainian exports to the customers around the world."

Ukrainian Foreign Minister Dmytro Kuleba said Monday marked a "day of relief for the world, especially for our friends in the Middle East, Asia, and Africa, as the first Ukrainian grain leaves Odessa after months of Russian blockade."

And the Kremlin hailed the "very positive" development, saying it was a "good opportunity to test the effectiveness of the mechanisms" agreed in in Istanbul.

The long-awaited consignment however is just the beginning and Ukraine Infrastructure Minister Oleksandr Kubrakov said 16 more ships -- blocked since the start of the invasion -- were "waiting for their turn".

"We are planning to reach full efficiency at of shipments of agricultural products during the following weeks," he added.

Ukraine had worked to boost grain deliveries by train, road and up the river Danube, but those efforts still left a large shortfall compared to what it was previously able to export by sea.

Although Monday was the first formal export of Ukrainian grain from Odessa, Kyiv has accused Russia repeatedly of seizing and illegally exporting cereals from regions it occupies.

Kherson counter-offensive 

The departure of the Razoni comes one day after Ukrainian agricultural magnate Oleksiy Vadatursky, 74, was killed by a Russian missile in the battle-scarred city of Mykolaiv in the south.

Vadatursky owned major grain exporter Nibulon and was previously decorated with the prestigious "Hero of Ukraine" award.

Mykolaiv -- which has been attacked frequently -- is the closest Ukrainian city to the southern front where Kyiv's forces are looking to launch a major counter-offensive to recapture territory lost after Russia's February invasion.

Ukraine said Monday its forces had retaken more than 40 settlements in the key southern region of Kherson.

The governor of Mykolaiv meanwhile said that three people had been injured in the city in "massive" Russian shelling overnight that damaged homes and damaged humanitarian supplies.

Despite progress on the grain exports, there was also Russian shelling in the war-scarred east of the country, where Russian troops have been fighting deeper into the Donbas region.

The head of the industrial Donetsk region, Pavlo Kyrylenko said Russian shelling over the past 24 hours had killed three people.

The Razoni's departure came after Russian authorities in the Crimean Black Sea peninsula -- seized by Moscow from Ukraine in 2014 -- said a small explosive device from a commercial drone, likely launched nearby, hit the navy command in Sevastopol.

The local mayor blamed "Ukrainian nationalists" for the attack that forced the cancellation of festivities marking Russia's annual holiday celebrating the navy.

Ukraine's navy accused Russia of staging the attacks as a pretext to cancel the festivities.