After penetrating a major line of Russian defences around the southern village of Robotyne, Ukrainian forces are now engaged in a fierce battle a few miles farther to the east as they seek to take the next step of a hard-fought counteroffensive, according to Ukrainian military commanders and western military analysts.

The Ukrainian 46th Brigade, which is taking part in the fighting in the area, said that its assault units were attacking Russian positions near the village of Verbove, 14km east of Robotyne.

Weeks of brutal fighting have resulted in small but significant advances that Ukrainian forces are trying to exploit. The move toward Verbove is notable because it shows that Ukraine feels it holds Robotyne securely enough to try to press forward.

While western analysts believe that Ukraine has broken through a first line of defence in Robotyne, they note that several other defensive lines converge around Verbove.

Ukrainian military officials have warned not to expect rapid progress and troops are fighting hard for every bit of ground retaken. There are still miles of formidable Russian defences — including mines, tank traps, fortified trenches, concrete pillboxes and sniper nests — ahead as Ukrainian troops seek to push south and drive a wedge between Russia and occupied Crimea.

On Thursday, the brigade said its forces had reached the western part of Verbove, though it underlined that there were much more difficult fights ahead.

“The battles will be for heights further south and southwest,” the brigade said. The brigade cautioned against “hype” since even if they punch through the next defensive line, Russia has more ahead.

At the same time, they also have to defend against Russian efforts to drive them back again.