Russia struck Ukrainian ports on Tuesday, a day after pulling out of a United Nations.-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 described a wave of missile and drone attacks on Ukraine’s ports as “mass revenge strikes” in retaliation for attacks by Ukrainian seaborne drones 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 U.N. 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 Russia’s main port, 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.

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 defense ministry said its forces had advanced 2 km (1.24 miles) in the vicinity of Kupiansk, a frontline railway hub recaptured by Ukraine in an offensive last year. Kyiv acknowledged heavy fighting in the area and said Russia was making a major push there. 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.

Moscow, for its part, said it had foiled a Ukrainian drone strike on Crimea, with no major damage on the ground, and had reopened a single lane of road traffic on the Crimea bridge.