Ukrainian forces have begun an offensive operation against Russian troops in the Kherson region - a key, Russia-occupied part of southern Ukraine, two Ukrainian sources have told Sky News.
There have been growing expectations that Ukrainian troops were planning to launch an operation to recapture Kherson, which was the first regional capital that Russia seized from Ukraine early in the invasion.
The first source said: "The Ukrainian operation in Kherson started."
The source said the Ukrainian armed forces were advancing and forcing the Russian side "to step back from their positions".
A second source said the push is taking place from several directions in the Kherson region.
"We started the offensive in a few directions. We can't give too many details about this operation but it has begun," the second source said.
Southern Command spokesperson Natalia Humeniuk confirmed the offensive in a news briefing and said it included the Kherson region, according to quotes from the Ukrainian broadcaster Suspilne.
She said Ukraine had struck more than 10 ammunition dumps in the past week, and they had "unquestionably weakened the enemy".
She declined to give exact details of the counter-offensive, saying Russian forces in southern Ukraine remained "rather powerful".
"The counter-offensive is already ongoing for a while (in the sense of) exhausting the enemy and not giving him an opportunity to advance," Ms Humeniuk told Suspilne, adding that the offensive phase of this had begun on Monday.
But the governor of Ukraine's Russian-annexed Crimea Peninsula, Sergei Aksyonov, dismissed her announcement as "another fake of Ukrainian propaganda".
However a tweet from the Ukraine government's Stratcom Centre said: "The Armed Forces of Ukraine have breached the occupiers' first line of defence near Kherson.
"They believe that Ukraine has a real chance to get back its occupied territories, especially considering the very successful use of Western weapons by the Ukrainian army."
Ukraine's presidential office reported heavy fighting and multiple Ukrainian strikes in the Kherson region, most of which is occupied by the Russians.
The Ukrainian strategy has focused on destroying four bridges which Russian forces must hold to supply Kherson, at the southern end of the Dnipro River.
It comes as a team from the UN nuclear watchdog - International Atomic Energy Agency (IAEA) - headed to Ukraine to inspect Europe's biggest nuclear plant, which was captured by Russian forces early on in the invasion but still run by Ukrainian workers.
Moscow and Kyiv have both accused each other of carrying out shelling near the Zaporizhzhia power station amid fears there could be a radiation leak.
With IAEA experts expected to arrive there later this week, the agency tweeted that the mission would look at physical damage, evaluate conditions for staff and "determine functionality of safety and security systems".
It would also "perform urgent safeguards activities", a reference to keeping track of nuclear material.
Russian-installed officials said on Monday that a Ukrainian missile strike had punched a hole in the roof of a fuel depot at the plant.
Russia's defence ministry claimed its forces had shot down a Ukrainian drone that was trying to attack the power station, according to Russian news agencies.
It reported there was no serious damage and radiation levels were normal.
The UN, US and Ukraine have called for the withdrawal of Russian military equipment and personnel from the complex so it is not a target - but the Kremlin insisted its troops will not leave.
The Kremlin also said the IAEA mission was "necessary" and urged the international community to pressure Ukraine to reduce military tensions at the plant.