More than 6.7 million people, mostly women and children, have fled Ukraine since the war broke out six months ago. Soon, these children who have taken refuge in nearby Romania will have school essentials, teachers and a safe learning environment thanks to generous Herald readers. Video / NZ Herald / World Vision

Ukraine claimed to have destroyed bridges and ammunition depots and pounded command posts in a surge of fighting in the Russian-occupied south, fuelling speculation that its long-awaited counteroffensive to try to turn the tide of war was under way.

Russia said it repelled the attack and inflicted heavy casualties.

The clashes were in the country's Kherson region, where Moscow's forces rolled up major gains early in the war. But Ukrainian authorities kept the world guessing about their intentions.

While independent verification of battlefield action has been difficult, Britain's Defence Ministry said in an intelligence report that several Ukrainian brigades had stepped up their artillery fire in frontline sectors across southern Ukraine.

The port city of Kherson, with a prewar population of about 300,000, is an important economic hub close to the Black Sea and the first major city to fall to the Russians in the war, which began six months ago. The port remains at the heart of Ukraine's efforts to preserve its vital access to the sea, while Russia views it as a key point in a land corridor extending from its border to the Crimean Peninsula, which it seized in 2014.

Occupation forces have spoken of plans to hold a referendum on making the Kherson region a part of Russia and have pressured residents to take Russian citizenship and stop using Ukraine's currency.

Ukraine's presidential office reported that "powerful explosions continued during the day and night in the Kherson region. Tough battles are ongoing practically across all" of the area. Ukrainian forces, the office said, destroyed ammunition depots and all large bridges across the Dnieper River vital to supplying Russian troops.

The Ukrainian military also reported destroying a pontoon bridge on the Dnieper that the Russian forces were setting up, and hitting a dozen command posts with artillery fire.

Russian state news agency Tass reported that explosions rocked Kherson on Tuesday morning — most likely caused by air defence systems.

Vladimir, 66, stands next to the wreckage of his house after being bombed by Russians in Chernihiv, Ukraine. Photo / Emilio Morenatti

Alluding to the talk of a major counteroffensive, Ukrainian President Volodymyr Zelenskyy said in his nightly video address on Monday that one "won't hear specifics from any truly responsible person" about Kyiv's intentions, "because this is war".

The British said that most of Russia's units around Kherson "are likely under-manned and reliant upon fragile supply lines" while its forces there are undergoing a significant reorganisation.

Russian Defence Ministry spokesman Lieutenant General Igor Konashenkov maintained that its forces stood up well and that Ukraine lost hundreds of troops, tanks and other armoured vehicles in Monday's action. His claim could not be independently verified.

A firefighter takes a break as he works to extinguish a fire after a Russian attack that heavily damaged a building in Sloviansk, eastern Ukraine. Photo / Leo Correa, AP

Ukrainian independent military analyst Oleh Zhdanov told The Associated Press that "it will be possible to talk about the effectiveness of Ukrainian actions only after large cities are retaken". He added that Ukrainian forces had breached the first and the second lines of defence in the Kherson region several times in the past, "but it didn't bring about results".

"The most important thing is Ukrainian artillery's work on the bridges, which the Russian military can no longer use," Zhdanov said.

The war has ground to a stalemate over the past months, 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, at least nine civilians were killed in more Russian shelling, Ukrainian officials said, from the Black Sea port of Mykolaiv to the northeastern industrial hub of Kharkiv, where five were killed in the city centre.

Amid fears that the fighting around the Russian-occupied Zaporizhzhia nuclear power plant could lead to a catastrophe, a UN atomic energy agency team set out on a mission to inspect and safeguard the complex. Ukraine and Russia have accused each other of shelling the area over and over.

Ukrainian President Volodymyr Zelenskyy, second left, talks with International Atomic Energy Agency Director General Rafael Gross in Kyiv. Photo / Ukrainian Presidential Press Office via AP

Nikopol, a city just across the Dnieper from the plant, again came under a barrage of heavy shelling, local authorities said, with a bus station, stores and a children's library damaged. And a Russian missile strike targeted the city of Zaporizhzhia, about 50km from the plant, Ukraine said.

In other developments:

• The first ship carrying grain from war-torn Ukraine for people in the hungriest parts of the world docked at the Horn of Africa port of Djibouti as deadly drought and conflict grip East Africa. The grain is going to Ethiopia.
• European Union ministers debated ways to ramp up weapons production, boost military training for Ukraine and inflict heavier costs on Russia.
• German Chancellor Olaf Scholz said his country is well prepared to tackle a possible energy shortage because of Russia's squeeze on European gas supplies. Russia has cut off or reduced the flow of natural gas to a dozen EU countries, raising fears ahead of winter.

- Paul Byrne, AP