• Ukraine used US-supplied armored vehicles to break through a series of key Russian defensive lines.

• The vehicles, called Strykers, were first spotted in battle last month.

• The success comes a day after Ukrainian missiles hit Russia's Black Sea Fleet headquarters in Crimea.

Ukraine used US-provided Stryker vehicles — like those pictured above — to break through a series of Russian defensive belts.Picture Alliance/Getty Images

Ukrainian officials said they've broken through a key line of Russian defense in the country's southeast thanks in part to US-provided armored vehicles.

Ukrainian forces broke through the Surovikin Line, a series of Russian defenses littered with anti-vehicle spikes and minefields, a commanding officer told The Washington Post. Ukraine first sent these US-supplied vehicles — called Strykers — into battle last month in the southern Zaporizhzhia Region.

The break in the line was small and met with heavy fire from Russia, The Wall Street Journal reported, but Ukrainian commanders are working to bring more vehicles to less fortified areas of the line.

"We are pushing through," a Ukrainian officer told the Journal.

The Institute for the Study of War said their ability to bring Strykers "to and through the most formidable Russian defenses" is an important sign of progress for the Ukrainian counteroffensive.

This announcement comes one day after a Ukrainian missile strike on Russia's Black Sea Fleet headquarters in Crimea killed nine Russian military personnel and wounded at least 16 others, Ukrainian intelligence chief Kyrylo Budanov said.