In a surge of fighting in the Russian-occupied south, Ukraine claimed to have destroyed bridges and ammunition depots and pounded command posts, fueling speculation Tuesday that its long-awaited counteroffensive to try to turn the tide of war was underway. Russia claimed to have repulsed the attack and suffered heavy casualties.

The fighting took place in the country’s Kherson region, where Moscow’s forces made significant gains early in the war. However, Ukrainian authorities kept the rest of the world guessing about their intentions.

While independent verification of battlefield action has been difficult, the United Kingdom’s Defense Ministry said in an intelligence report that several Ukrainian brigades had increased artillery fire in front-line sectors across southern Ukraine.

Kherson, with a prewar population of around 300,000, is an important economic hub close to the Black Sea and the first major city to fall to the Russians in the six-month-old war. The port remains central to Ukraine’s efforts to maintain vital sea access, while Russia sees it as a key point in a land corridor stretching from its border to the Crimean Peninsula, which it seized in 2014.

Occupation forces have discussed holding a referendum on annexing the Kherson region to Russia, as well as pressuring residents to take Russian citizenship and stop using Ukrainian currency.

According to Ukraine’s presidential office, “powerful explosions continued throughout the day and night in the Kherson region.” Tough battles are raging practically everywhere” in the area. According to the office, Ukrainian forces destroyed ammunition depots as well as all large bridges across the Dnieper River that were critical to supplying Russian troops.

The Ukrainian military also claimed to have destroyed a pontoon bridge on the Dnieper that Russian forces were erecting and to have hit a dozen command posts with artillery fire.

According to the Russian state news agency Tass, explosions rocked Kherson on Tuesday morning, most likely caused by air defense systems.

In an apparent reference to talk of a major counteroffensive, Ukrainian President Volodymyr Zelenskyy said in his nightly video address on Monday that “no truly responsible person” will “hear specifics from any truly responsible person” about Kyiv’s intentions, “because this is war.”

According to the British, most of Russia’s units near Kherson “are likely under-manned and reliant on fragile supply lines,” while its forces in the region are undergoing significant reorganization.

Lt. Gen. Igor Konashenkov, spokesman for the Russian Defense Ministry, insisted that Russia’s forces held up well and that Ukraine lost hundreds of troops, tanks, and other armored vehicles in Monday’s action. His claim was not independently verifiable.

According to Ukrainian independent military analyst Oleh Zhdanov, “it will be possible to talk about the effectiveness of Ukrainian actions only after large cities are retaken.” He added that Ukrainian forces had previously breached the first and second lines of defense in the Kherson region, “but it did not result in results.”

“The most important thing is the work of Ukrainian artillery on bridges that the Russian military can no longer use,” Zhdanov said.

Over the past few months, the war has come to a halt, with casualties and destruction rising and the population bearing the brunt of the suffering during relentless shelling in the east and south. In other battlefield reports, Ukrainian officials said at least nine civilians were killed in more Russian shelling, from the Black Sea port of Mykolaiv to the northeastern industrial hub of Kharkiv, where five were killed in the city center.

Fearing that the fighting around the Russian-occupied Zaporizhzhia nuclear power plant would result in a disaster, a team from the United Nations Atomic Energy Agency set out on a mission to inspect and safeguard the complex. Ukraine and Russia have repeatedly accused each other of shelling the area.

Local authorities reported that a bus station, stores, and a children’s library were damaged in Nikopol, a city just across the Dnieper from the plant. And a Russian missile strike targeted the city of Zaporizhzhia, about 50 kilometers (30 miles) from the plant, Ukraine said.