Russia has suspended participation in a UN-brokered deal to export agricultural produce from Ukrainian ports after attacks on ships in Crimea, Russia's defence ministry said, dealing a blow to a three-month agreement aimed at easing a global squeeze on grain supplies.

Russia said that Ukrainian forces, with the help of drones, attacked ships from the Black Sea Fleet in Sevastopol, the biggest city in Russian-annexed Crimea, in the early hours of this morning.

"Taking into account... the terrorist act by the Kyiv regime with the participation of British experts against the ships of the Black Sea Fleet and civilian vessels involved in ensuring the security of the 'grain corridor', the Russian side suspends participation in the implementation of agreements on the export of agricultural products from Ukrainian ports," the ministry said in a statement.

The ministry said earlier that the drone attacks were largely repelled, with minor damage to a Russian minesweeper.

The Ukrainian president's chief of staff, Andriy Yermak, accused Russia of "blackmail" and "invented terror attacks" on its own territory following explosions in the Crimea peninsula.

His comments were an apparent response to Russian accusations that Ukraine was behind the blasts.

Britain has bluntly rejected Russian claims its specialists were involved in the attacks, saying "the Russian Ministry of Defence is resorting to peddling false claims of an epic scale."

The British defence ministry said that this "invented story says more about arguments going on inside the Russian Government than it does about the West".

Ukraine has criticised Russia's decision to pull out of the grain deal, saying that Moscow's blaming a drone attack was a "false pretext".

"Moscow is using a false pretext to block the grain corridor that ensures food security for millions of people. I call on all states to demand that Russia stop its hunger games and recommit to fulfilling its obligations," Ukrainian Foreign Minister Dmytro Kuleba said on Twitter.

The United Nations has also called for the preservation of the agreement.

"It is vital that all parties refrain from any action that would imperil the Black Sea Grain Initiative which is a critical humanitarian effort," said Stephane Dujarric, spokesman for the UN secretary-general, underscoring the initiative was "having a positive impact" on food access for millions worldwide.

He added that the UN chief's office was in touch with Russian authorities over the issue.

The grain agreement signed by Russia and Ukraine on 22 July in Istanbul allowed the export of more than nine million tonnes of grain and agricultural products and is supervised by a body in Turkey.

Turkey has not been "officially notified" by Russia of withdrawal from the Ukraine grain deal, of which Ankara and the United Nations are guarantors, a Turkish security source told AFP.

Separately, Russian Agriculture Minister Dmitry Patrushev said earlier that Russia was ready to supply up to 500,000 tonnes of grain to poor countries in the next four months for free, with assistance from Turkey, and supplant supplies of Ukrainian grains.

"Taking into account this year's harvest, the Russian Federation is fully prepared to replace Ukrainian grain and deliver supplies at affordable prices to all interested countries," he said.

Since Russia and Ukraine signed the UN-backed Black Sea Grain Initiative in Turkey on 22 July, several million tonnes of corn, wheat, sunflower products, barley, rapeseed and soya have been exported from Ukraine.

United Nations aid chief Martin Griffiths had said on Wednesday that he was "relatively optimistic" that the deal would be extended beyond mid-November.

Under the agreement, Ukraine was able to restart its Black Sea grain and fertilizer exports, which had stalled when Russia invaded its neighbor on 24 February. The Ukraine export deal was initially agreed for 120 days.

In a separate statement, the Russian foreign ministry said that following the attacks on Russian ships "the Russian side cannot guarantee the safety of civilian dry cargo ships participating in the "Black Sea Initiative", and suspends its implementation for an indefinite period.

"Relevant instructions were given to Russian representatives at the Joint Coordination Center in Istanbul, which controls the transportation of Ukrainian food," it added, referring to Turkey's role in implementing the deal.

Late yesterday a spokesman for UN Secretary-General Antonio Guterres had appealed to the parties to renew the pact.

'Most massive' attack peninsula has seen

Mikhail Razvozhayev, the Moscow-installed governor of Sevastopol, said the drone attack was the "most massive" the peninsula had seen.

The city's services were on "alert", but he claimed no "civilian infrastructure" had been damaged.

He called on residents of the city not to post videos of the incident on social media.

"It should be clear to everyone that such information is much needed for Ukrainian Nazis in order to understand how the defence of our city is built," he said.

City authorities said the harbour was "temporarily" closed to boats and ferries and urged people "not to panic".

Attacks on Crimea, annexed by Moscow in 2014, have increased in recent weeks, as Ukraine presses a counter-offensive in the south to retake territory held by Moscow for months.

Moscow-installed authorities in Kherson, just north of Crimea, have vowed to turn the city into a fortress, preparing for an inevitable assault.

On Thursday, Mr Razvozhayev said a thermal power station had been attacked in Balaklava, in the Sevastopol area.

He claimed there was only minor damage and no casualties.

In early October, Moscow's bridge linking Crimea to the Russian mainland - personally inaugurated by President Vladimir Putin in 2018 - was damaged by a blast that Putin blamed on Ukraine.

The Russian fleet stationed in the port had also been attacked by a drone in August.

Russia's allegations came as the Ukrainian army reported fighting in the Lugansk and Donetsk regions in the east, including near Bakhmut - the only area where Moscow's forces have advanced in recent weeks.

Pro-Russian separatists fighting alongside Moscow also announced a new prisoner exchange with Kyiv, saying 50 will return home from each side.

On the southern front, AFP journalists witnessed artillery battles in the village of Kobzartsi, the last settlement on the Ukrainian side before the line of contact with the Russians.

Both sides were gearing up for the battle for the city of Kherson, the regional capital that fell to Moscow's forces in the first days of their offensive.