Russia 'open' to grain deal talks: Putin

President Vladimir Putin said Monday that Russia was "open" to talks on restoring the landmark Black Sea grain export deal, hours after Moscow pounded one of Ukraine's key grain-exporting ports overnight.

The UN and Turkey-brokered deal, which aimed to ensure safe navigation for civilian ships through the Black Sea, collapsed after Russia pulled out in July.

Moscow has since repeatedly attacked Ukraine's port infrastructure in what Kyiv says is a cynical attempt to damage its exports and undermine global food security.

The Russian drone attack on Monday damaged a grain export hub on the Danube river, officials said, adding that warehouses and agricultural equipment were damaged.

The regional governor Oleg Kiper announced that air defence systems had downed 17 drones.

Hours later Putin met his Turkish counterpart Recep Tayyip Erdogan in Sochi, amid hopes the two could restore the deal, with Erdogan promising he would make a "very important" announcement on grain exports after the talks.

"I know you intend to raise the issue of the grain deal. We are open to negotiations on this issue," Putin said in televised comments alongside Erdogan.

Ukraine's foreign ministry claimed Monday that some of the drones in the attack had fallen on Romanian territory, although Bucharest "categorically" denied the claim.

While most fighting during Russia's offensive has taken place within Ukraine's borders, Kyiv has occasionally claimed Russian munitions have fallen into European countries, claims that its NATO allies have largely dismissed.

"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 claiming the 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 offensive.

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.

His departure comes during a highly-scrutinised Ukrainian counteroffensive in the south and east of the country, which officials said Monday had made limited progress.

"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.

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 US-made Ukrainian military boats carrying troops in the Black Sea.

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