Ukrainian fighters on the frontline say troops have not only crossed into Russian occupied territory but held a position, apparently for the first time, on the fiercely defended east (or left) bank of the Dnipro River in Kherson region.
The development is potentially significant. Ukraine's counteroffensive aims to slice through Russian occupied territory, severing a land corridor to the Crimean peninsula which Moscow annexed - illegally - in 2014.
In a text exchange, the 46th brigade told the BBC that troops were engaged in heavy fighting as they try to take full control of the village of Krynky.
If successful, the force said, the settlement would give advanced units a base from which to launch a larger offensive aimed at dividing Russian troops and cutting off their supply lines.
The US-based Institute for the Study of War (ISW) said that Ukrainian forces were continuing larger than usual ground operations on the east bank and Russian military bloggers have also noted fighting in Krynky.
And the action is being closely scrutinised - success here would be seized upon by Ukraine's military chiefs. Their counteroffensive, launched in June, has made slow progress, with limited territorial gains.
But the fighters we spoke to acknowledged that, even if the troops were to take Krynky, a modern day "Normandy landings" style attack was unlikely.
And they gave us a glimpse of the dangers and challenges they face, particularly as winter approaches.
The 46th brigade is fighting further along the vast frontline in the Zaporizhzhia region with the same aim in mind - to cut off Russian access to the Crimean peninsula.
They told us they had just managed to partially breach the first line of Russian defence near the village of Verbove - and that they'd successfully targeted some ammunition depots and bases, but that progress was limited.
They described heavily mined Russian fortifications and daily air attacks on their logistics routes without, they complained, air support of their own.
Russian troops, they said, had been on the defensive but were now on the attack.
And, as winter approaches, conditions are deteriorating - for both sides.
Rainy weather is affecting the work of drones, reconnaissance equipment and aviation, the soldiers said. But they did not anticipate any let-up in the fighting.
"Winter will not be a time for respite," they said.
Additional reporting by Anastasiia Levchenko