(AFP) - Russia damaged a grain export hub in a massive drone attack, Ukrainian officials said Monday, as Turkish leader Recep Tayyip Erdogan arrived in Moscow hoping to revive the deal allowing Ukraine to export grain through the Black Sea.

Ukraine's foreign ministry said the drones had fallen on Romanian territory and released images of explosions across the Danube river but Bucharest "categorically" denied the claim.

The strikes in Ukraine's Odesa region came hours before Russian President Vladimir Putin hosted Erdogan, who hopes to revive a grain deal aiming to safeguard Ukraine's Black Sea exports that Moscow exited in July.

"Seventeen drones were shot down by our air defence forces," Odesa governor Oleg Kiper wrote on social media.

He said there were no casualties but acknowleged infrastructure had been hit, including warehouses, agricultural machinery and industrial equipment.

The Danube river port of Izmail, which borders NATO member Romania, has become a main export route for Ukrainian products following Russia's withdrawal from the UN-brokered grain deal.

"Russian 'Shaheds' fell and detonated on Romanian territory overnight," Kyiv's foreign ministry spokesman Oleg Nikolenko said on social media.

A spokesman for the State Border Guard Service told AFP that two detonations were observed.

But NATO member Romania categorically denied that Russian drones fell on its soil during the strike.

- Defence minister resigns -

"At no time did the means of attack used by the Russian Federation pose a direct military threat to Romania's national territory or territorial waters," the Romanian defence ministry said.

Ukraine's military said Russia had used Iranian-made Shahed drones in the "massive" overnight attack.

The strikes come a day after Ukraine fought off a barrage of Russian drones in the same region, with Russia's army saying those assaults targeted fuel storage facilities in the nearby port of Reni.

Ukrainian Defence Minister Oleksiy Reznikov meanwhile announced on Monday he had handed in his resignation to parliament after Ukrainian President Volodymyr Zelensky called for "new approaches" to face Russia's invasion.

Zelensky's decision to remove Reznikov comes after several corruption scandals rocked the defence ministry, and the Ukrainian leader removed senior military recruitment officials across the country.

"It was an honour to serve the Ukrainian people and work for the (Ukrainian army) for the last 22 months, the toughest period of Ukraine's modern history," he added.

Zelensky's decision to replace his defence minister comes during a highly-scrutinised Ukrainian counteroffensive in the south and east of the country, which officials said Monday had made limited progress over the last week.

"The enemy is on the defensive in the Zaporizhzhia and Kherson sectors," Deputy Defence Minister Ganna Malyar said, referring to two southern regions that Moscow claimed to have annexed last year.

- Black Sea tensions rise -

She added that Ukrainian forces had also captured three square kilometres (around one square mile) near Bakhmut, a town in the east captured by Russia this May.

Earlier on Monday, Russia said it destroyed four Ukrainian military boats carrying troops in the Black Sea.

"Naval aviation aircraft of the Black Sea Fleet destroyed 4 'Willard Sea Force' US-made high-speed military boats with landing groups of the Armed Forces of Ukraine," Russia's defence ministry said on Telegram.

The ministry said the boats were "travelling in the direction of Cape Tarkhankut on the Crimean coast", without providing further details.

In a similar attack on August 30, Russia said its forces destroyed four Ukrainian military boats carrying up to 50 soldiers in the Black Sea.

In the early hours of Monday, the ministry said it had repelled a separate Ukrainian attack over the Black Sea.

Crimea was last attacked on Saturday, when Russia destroyed three Ukrainian naval drones that targeted the bridge connecting the peninsula to the Russian mainland.

Crimea, annexed by Russia in 2014, has been targeted by Kyiv throughout Moscow's Ukraine offensive but has recently come under more intense, increased attacks.

Kyiv has repeatedly said it aims to take back Crimea.

On August 24, Ukraine said it flew its flag on Crimea, in a symbolic move during a "special operation" to mark its second wartime Independence Day.

© Agence France-Presse