Ukraine's military has launched attacks on occupying Russian forces in the key southern Zaporizhzhia region, Russian officials and military bloggers say.
They say Ukrainian troops - backed by tanks, artillery and drones - are trying to advance south of the town of Orikhiv for the second night running.
A senior Ukrainian defence official said the enemy was in "active defence".
Several military experts have said the focus of Ukraine's long awaited counter-offensive will be Zaporizhzhia.
They argue Kyiv is trying to regain access to the Sea of Azov, splitting the occupying Russian forces in the region into two detached groupings.
That would not only weaken Russia's combat capability but also eliminate a land bridge to Crimea, the southern peninsula illegally annexed by Russia in 2014.
Ukraine has been planning a counter-offensive for months, but it has wanted as long as possible to train troops and to receive advanced military equipment from Western allies.
The government is deliberately saying little about its plans but its forces are now probing Russian positions at several points along the front line, looking for signs of weakness.
Meanwhile Russian attacks on Ukraine continue. Overnight it launched fresh cruise missile and drone strikes, with falling debris killing at least one person in Zhytomyr to the west of the capital Kyiv.
Russia seized most of the Zaporizhzhia region soon after President Vladimir Putin ordered a full-scale invasion in February 2022. However the regional capital, the city of Zaporizhzhia, is controlled by Ukraine.
The recent fierce fighting in the area continued overnight, with several Russian pro-Kremlin military bloggers reporting late on Thursday that Ukraine had resumed attacks, noting the movement of armoured personnel carriers and drones directing artillery fire.
Vladimir Rogov, a Russian-installed local official, said Russian troops were holding their positions amid "active fighting" towards the city of Tokmak, seen as a key target for Ukraine as it seeks to advance to the Sea of Azov.
It is difficult to verify the competing claims by both sides. Earlier on Thursday Ukraine's Deputy Defence Minister Hanna Maliar said Russian forces were currently "in active defence" in Zaporizhzhia and claimed advances were being made around the eastern city of Bakhmut, which has been largely destroyed during months of fighting.
The Zaporizhzhia region is also home to Europe's largest nuclear power plant, which is in an area controlled by Russian forces.
The plant is continuing to receive cooling water from the reservoir of the Nova Kakhovka dam on the Dnipro river, despite the dam's destruction on Tuesday which has seen water levels in the reservoir fall and large areas downstream of the dam flooded.
However the resulting emergency is threatening the region's water supplies, with the WHO also warning that cholera could spread.
It is also predicted to have serious long-term consequences for agriculture across one of the country's most fertile areas.