A powerful attack on a Russian air base in occupied Crimea was carried out by Ukrainian special forces, a Ukrainian government official told The Washington Post on Wednesday, implying that covert forces operating deep behind enemy lines will play an increasingly important role as the country expands its efforts to expel Russian troops.

The official, who spoke on the condition of anonymity because they were not authorized to speak publicly about the matter, did not reveal specifics about Tuesday’s attack. The attack represents a significant escalation in the nearly six-month-old conflict, demonstrating Ukrainian forces’ new ability to carry out attacks further behind the front lines than was previously thought possible.

At least three explosions shook the Saki air base in the Crimean Peninsula, which Moscow annexed in 2014 and is a popular vacation destination for Russians.

The Ukrainian air force said in a statement Wednesday that nine Russian military planes were destroyed at the base, which authorities say Russia used to launch missile strikes against Ukrainian-controlled territory.

If confirmed, the losses would be the highest for the Russian air force in a single day since the beginning of the war, and could limit Russia’s ability to launch missile strikes against Ukrainian cities in the short term. Throughout the war, Russia’s air force has struggled to mount airborne operations, and the majority of its strikes have been launched from planes and ships comfortably out of reach of Ukrainian air defenses in Russia or Russian-occupied territory.

The attack could also have a long-term impact on Russia’s ability to maintain control over southern Ukraine, particularly the nearby province of Kherson, where Ukrainian forces hope to launch a counteroffensive to reclaim territory. Crimea has emerged as a critical logistical hub for Russian operations in the south, serving as the primary transit point for troops, equipment, and ammunition reinforcing and supplying the Kherson region.

If Crimea is now in play and the peninsula is vulnerable to further attack, Russia will be forced to defend a part of Ukraine where it previously assumed it could operate freely. Andriy Zagorodnyuk, a former Ukrainian defense minister who is now chairman of Ukraine’s Center for Defense Strategies, an independent think tank, believes this could impede its ability to reinforce and supply its troops in the south.

What caused the explosions at the base is still unknown, and Ukraine, if it was responsible, may never publicly reveal its methods. According to Russian officials, the first known strike against Crimea was carried out by a drone less than two weeks ago, against the Russian naval base in Sevastopol. Some military experts believe Tuesday’s attack was also carried out by a drone, possibly operated by covert forces hidden nearby.

According to Oleksiy Danilov, secretary of Ukraine’s National Security and Defense Council, the shadowy force, led by Ukraine’s special operations forces and under the overall command of the Ukrainian army, has been stepping up activities aimed at disrupting Russia’s logistics, administration, and command and control.

Guerrilla activity has been particularly intense in the southern Kherson region, which borders Crimea. A Russian-installed official in Kherson was killed in a mysterious bomb blast on his way to work in June, one of several assassinations blamed on the resistance. According to images posted on social media, Ukrainian resistance groups posted posters around the city warning those working with the Russians that partisans would come after them as well.

The Saki air base is over 140 miles from the nearest front line, and US officials have told The Post that no US weapons were used in the attack, ruling out the possibility that HIMARS was to blame. According to Ukrainian officials, resistance operatives have assisted in locating key targets such as ammunition depots and command and control posts, as well as providing coordinates for HIMARS teams to target.

According to Russian officials, the blast on Tuesday was caused by an ammunition explosion, and at least one person was killed and 13 others were injured, including two children. On Wednesday, the Russian Defense Ministry did not issue an update to its statement.