The Ukrainian commander points ahead. There's a bridge over the Donets River, then forest. Beyond it lies the Russian city of Belgorod.

"That green hill over there is Russia," says commander Roman Gryshchenko.

In September, he helped push Moscow's forces back across the border near Kharkiv.

The retreat marked one of the high points of the Ukrainian counteroffensive in the northeast.

Now, Kyiv's troops in the village of Starytsya stand so close to Russia that some phones mistakenly get texts to say they've arrived in the country.

Gryshchenko insists the area is now "absolutely secure" -- but his troops are keeping watch.

They still face drone attacks, occasional artillery strikes and the threat of raids from Russian commandos, he says.

The 5,000-strong 127th brigade he leads was formed from a volunteer force, and its mission is to defend positions in this recaptured border zone.

The last official Ukrainian outpost is buried in a trench off a muddy road leading straight to Russia. Beyond it, special forces soldiers and border guards operate unseen.

A lone sentry keeps his finger on the trigger, alert to suspicious noises in the forest.

Below ground, a sheltered room with internet connection acts as a digital surveillance station.

Soldier Sheleh, 32, taps a computer mouse, its red light glowing in the dim room.

He inspects video footage of the area -- mostly bushes and dirt roads.

"Here we monitor our side of the border and watch for possible crossings that could be used for infiltration," he says.

They work day and night, and through bombardments, he adds.

That morning, a dozen Russian soldiers moved inside Ukrainian territory before a brigade drone dropped a grenade and forced them to retreat, according to a video the 127 brigade sent to AFP.

This area was among the first to be breached by columns of tanks sent through Belgorod in February.

With Ukraine's troops now in control, Gryshchenko takes a tour through the recaptured territory, revisiting old battle scenes.

A Russian camp abandoned in the panic of the retreat, with pieces on a chess board still frozen mid-game.

A stretch of river used by the Ukrainian forces to surprise Russian special forces in May.

And a charred tank, the first Russian T-90 destroyed in Ukraine.

For the commander, it's a trophy. But he regrets that it was so badly damaged his troops couldn't reuse it.

"If we'd had the weapons we do now in March, I'd already be on the Red Square" in Moscow, he said.