Russia has withdrawn from a year-old UN-brokered deal that lets Ukraine export grain through the Black Sea, a day after a blast believed to be ordered by Kyiv destroyed a key link with Crimea.

Russia’s move, which has caused fears it will drive price rises and put food out of reach for some, came late on Monday (local time).

It was just hours after the blast knocked out Russia’s Kerch Bridge in what Moscow said was a strike by Ukrainian sea drones. Two people died in the strike, and their young daughter was injured.

Moscow said it was a terrorist attack on the road bridge, which is a major artery for Russian troops fighting in Ukraine.

The Kremlin said there was no link between the attack and its decision to suspend the grain deal, over what it called a failure to meet its demands to implement a parallel agreement easing rules for its own food and fertiliser exports.

“Unfortunately, the part of these Black Sea agreements concerning Russia has not been implemented so far, so its effect is terminated,” Kremlin spokesman Dmitry Peskov said.

UN secretary-general Antonio Guterres signalled that Russia’s withdrawal meant the related pact to assist Russia’s grain and fertiliser exports was also terminated.

“Today’s decision by the Russian Federation will strike a blow to people in need everywhere,” he said.

Moscow said it would consider rejoining the grain deal if it saw “concrete results” on its demands but its guarantees for the safety of navigation would meanwhile be revoked.

In Washington, the White House said Russia’s suspension of the pact “will worsen food security and harm millions” and US Secretary of State Antony Blinken called it unconscionable.

Ukraine and Russia are some of the world’s biggest exporters of grain and other foodstuffs and any interruption could drive up food prices across the globe.

Shashwat Saraf, the emergency director in East Africa for the International Rescue Committee, said the effect would be profound in Somalia, Ethiopia and Kenya, which face the Horn of Africa’s worst drought in decades.

Ukraine’s President Volodymyr Zelensky raised the prospect of resuming grain exports without Russia’s participation, suggesting Kyiv would seek Turkey’s support to effectively negate the Russian de facto blockade imposed last year.

“Ukraine, the UN and Turkey together can ensure the operation of a food corridor and vessel inspections,” Mr Zelensky said in his nightly video message. He said the world “has the opportunity to show that blackmail is not allowed … We must all ensure security, protection from Russian madness”.

Ukrainian forces have been striking Russian supply lines as it pursues a counteroffensive to drive Russian forces out of its south and east.

On Monday it reported two more civilians killed by Russian forces, which it said had begun a major push in the north-east.

“For two days running, the enemy has been actively on the offensive in the Kupiansk sector in Kharkiv region. We are defending. Heavy fighting is going on and the positions of both sides change dynamically several times a day,” Deputy Defence Minister Hanna Maliar wrote on Telegram.

The armed forces said Russia had amassed a huge array of forces.

“In the Lyman-Kupiansk sector the enemy has concentrated a very powerful grouping. More than 100,000 personnel, more than 900 tanks, more than 555 artillery systems, and 370 multiple launch rocket systems,” Serhiy Cherevatyi, spokesman for the Ukrainian Armed Forces East group, said on national TV.

Reuters was not able to verify the accounts and there was no immediate comment from Russia.

The blast on the road bridge to Crimea could limit Moscow’s ability to supply its troops in southern Ukraine, although Russian President Vladimir Putin said the bridge had not been used for military transports for a long time.

Partial road traffic had been restored, Russian Deputy Prime Minister Marat Khusnullin said on Telegram.

Earlier, images showed part of the road bridge had come down and traffic halted in both directions, although a parallel railway bridge was still operational.

Blasts were reported before dawn on the 19-kilometre bridge, which Mr Putin ordered built after seizing and annexing Crimea from Ukraine in 2014.

Mr Putin told officials Russia would respond to the “senseless” attack.

-with AAP