Russia struck Ukrainian ports on Tuesday, a day after pulling out of a UN-backed deal to let Kyiv export grain, and Moscow claimed gains on the ground in an area where Ukrainian officials said Russian forces were going back on the offensive.

Russia said it hit fuel storage in Odesa and a plant making seaborne drones there, as part of “mass revenge strikes” in retaliation for attacks by Ukraine that knocked out its road bridge to the occupied Crimean Peninsula.

Shortly after the bridge was hit on Monday, Moscow withdrew from a year-old UN-brokered grain export deal, a move the United Nations said risked creating hunger around the world.

Falling debris and blast waves damaged several homes and unspecified port infrastructure in one of Ukraine’s main ports, Odesa, according to Ukraine’s southern operational military command. Local authorities in Mykolaiv, another port, described a serious fire there.

The Russian attacks on ports provide “further proof that the country-terrorist wants to endanger the lives of 400 million people in various countries that depend on Ukrainian food exports”, said Andriy Yermak, the head of Ukraine’s presidential staff.

Ukraine’s air force said six Kalibr missiles and 31 out of 36 drones were shot down. Moscow, for its part, said it had foiled a Ukrainian drone strike on Crimea, with no major damage on the ground, and reopened a single lane of road traffic on the Crimea bridge.

Six weeks since Ukraine launched a counteroffensive in the east and south, Russia is mounting a ground offensive of its own in the northeast.

Russia’s defence ministry said its forces had advanced 2 km (1.2 miles) in the vicinity of Kupiansk, a frontline railway hub recaptured by Ukraine in an offensive last year. Kyiv acknowledged a “complicated” situation in the area. Reuters could not independently verify the situation.

Since Ukraine began its counteroffensive last month, Kyiv has recaptured some villages in the south and territory around the ruined city of Bakhmut in the east, but has yet to attempt a major breakthrough across heavily defended Russian lines.

‘A blow to people in need’

The Black Sea grain export deal brokered a year ago by Turkey and the United Nations was one of the only diplomatic successes of the war, lifting a de facto Russian blockade of Ukrainian ports and heading off a global food emergency.

Ukraine and Russia are both among the world’s biggest exporters of grain and other foodstuffs. If Ukrainian grain is again blocked from the market, prices could soar around the world, hitting the poorest countries hardest.

“Today’s decision by the Russian Federation will strike a blow to people in need everywhere,” UN Secretary-General Antonio Guterres said on Monday.

Moscow spurned calls from Ukraine to allow shipping to resume without Russian participation, with the Kremlin openly saying ships entering the area without its guarantees would be in danger.

“We’re talking about an area that’s close to a war zone,” Kremlin spokesman Dmitry Peskov said. “Without the appropriate security guarantees, certain risks arise there. So if something is formalised without Russia, these risks should be taken into account.”

Russia says it could return to the grain deal, but only if its demands are met for rules to be eased for its own exports of food and fertiliser. Western countries call that an attempt to use leverage over food supplies to force a weakening in financial sanctions, which already allow Russia to sell food.

Ukraine’s President Volodymyr Zelenskiy has called for the grain deal to continue without Russia, effectively seeking Turkey’s backing to negate the Russian blockade. Turkish President Tayyip Erdogan, the deal’s sponsor, says he thinks Moscow can be persuaded to return.

Any attempt to reopen Ukrainian grain shipments without Russia’s participation would depend on insurance companies agreeing to provide coverage. Industry sources have told Reuters they are considering the implications.

Slow counteroffensive

Russia’s claim on Tuesday to have advanced around Kupiansk was a rare sign of Moscow attempting go back on the offensive since Kyiv launched its counteroffensive last month.

Both sides have endured bitter losses in Europe’s bloodiest combat since World War Two, yet front lines have moved only incrementally since last November, despite a massive Russian winter offensive followed by Ukraine’s counterassault.

“For two days running, the enemy has been actively on the offensive in the Kupiansk sector in Kharkiv region,” Ukraine’s Deputy Defence Minister Hanna Maliar wrote on Telegram.

“We are defending. Heavy fighting is going on and the positions of both sides change dynamically several times a day.”

Oleksander Syrskyi, the commander of Ukraine’s ground forces, described the situation in that area as “complicated but under control”. Serhiy Cherevatyi, spokesperson for Ukraine’s eastern grouping of forces, said the Russian military had amassed more than 100,000 troops and more than 900 tanks in the area.

Ukraine’s counteroffensive has made limited gains near Bakhmut and along two major axes in the south, but its assault force equipped with billions of dollars worth of new Western weapons and ammunition has yet to confront the main Russian defensive line.

Kyiv says it is deliberately advancing slowly to avoid high casualties on fortified defensive lines strewn with landmines, and is focused for now on degrading Russia’s logistics and command. Moscow says the Ukrainian counteroffensive has failed.