KYIV, Ukraine>> For weeks, Ukrainian forces have been probing Russian defenses in the southeast, looking for an opening to push their armored vehicles behind the main Russian line. But artillery fire and Russian counterattacks had been too intense to allow Ukrainian armor to pass.

This week, though, Ukrainian armored vehicles advanced past Russia’s main anti-tank defenses at one location on the front line, according to reconnaissance video and commanders, showing slight progress in Kyiv’s halting counteroffensive.

Lt. Ashot Arutiunian, the commander of a drone reconnaissance unit operating in the area, said the vehicles had broken through near Verbove, a village in the Zaporizhzhia region. The vehicles, however, are confined to slender routes through minefields and have little room to maneuver, he said.

Earlier this summer, Ukrainian tanks had broken through a less formidable layer of defenses, and infantry had pierced a second line, seen as Russia’s main anti-tank barrier.

The breach, a swath of farm fields and tree thickets 1 mile or so wide, is a crucible of concentrated Russian and Ukrainian artillery fire, littered with blown-up vehicles and dotted with craters. It sits near Robotyne, a village that Ukrainian forces captured several weeks ago.

The recent breakthrough, shown in unverified videos published Thursday, suggested that weeks of bloody infantry fighting had secured the breach sufficiently to move heavy weaponry forward. That would be a positive step for Ukraine in its campaign to drive Russian forces from the south. More Russian minefields and anti-tank barriers remain ahead.

The breach of defenses in the Zaporizhzhia region was first reported Thursday by The Wall Street Journal.

It is unclear whether the Ukrainian vehicles that breached the defenses can gain control of their new positions securely enough to be based behind Russian lines. Doing so might allow them to push deeper or turn to attacking the Russians on the rear-facing edge of their trenches, which are more lightly defended.

“They are beyond the dragon’s teeth,” Arutiunian said of Ukrainian armored vehicles, using the nickname for a type of triangular, concrete anti-tank barrier the Russians deployed in southern Ukraine. “It is beautiful; it is important. But more important is whether we are able to dig into these positions.”

Soldiers can clear paths through the dragon’s teeth obstacles by moving them, but this is difficult to do while under fire and if the area is mined. It is unclear exactly how the Ukrainians broke through.

A Ukrainian military intelligence officer, who asked to remain anonymous because he was not authorized to comment publicly, confirmed that Ukrainian vehicles had crossed the dragon’s teeth barrier near Verbove, but he played down the significance.

“It’s too early for conclusions” on where the advance with heavy weapons might lead, he said. “The breakthrough for now is small. There is still a lot of work,” he added, noting that the Russians have moved reinforcements to the area.

One video — verified by the Institute for the Study of War, an American analytical group providing updates on the fighting — showed tracked vehicles driving in a field amid explosions in an area past the Russian line.Although Russian forces have multiple layers of defense, the line that passes near Verbove is a significant barrier to tanks. Ukraine’s military has said the Russian defenses ahead are less formidable.

Meanwhile The Associated Press report that Ukraine struck the headquarters of Russia’s Black Sea Fleet in a missile attack Friday that left a serviceman missing and the main building smoldering, according to military officials on both sides of the war and images from the scene in Crimea.

Ukraine has increasingly targeted naval facilities in Crimea in recent weeks while the brunt of its summer counteroffensive makes slow gains in the east and south of Ukraine, the Institute for the Study of War said Thursday. Military experts say it is essential for Ukraine to keep up its attacks on targets in Crimea to degrade Russian morale and weaken its military.

In Canada, President Volodymyr Zelenskyy spoke to a special joint session of Canada’s House of Commons and Senate. The audience, which included guests as well as lawmakers, repeatedly interrupted Zelenskyy with standing ovations.

“It is genocide, what Russian occupiers are doing to Ukraine,” Zelenskyy said. “It is not just about an ordinary conflict. It is about saving lives of millions of people.”