According to European intelligence officials and defense analysts, explosions at a Russian airbase in Crimea that Ukraine claims destroyed nine fighter aircraft may indicate new Ukrainian offensive capabilities that complicate Kremlin efforts to support its invading forces.

“The occupiers lost ten combat aircraft in a single day, nine in Crimea and one more in the direction of Zaporizhzhia,” Ukrainian President Volodymyr Zelenskiy said in his nightly video address to the nation. More Russian armored vehicles, ammunition warehouses, and logistics routes were also destroyed, he said, adding that “the more losses the occupiers suffer, the sooner we will be able to liberate our land.”

The precise nature of Tuesday’s explosions is unknown, as Russian officials blame munitions safety lapses and deny any Ukrainian role, while Ukrainian officials hint at their involvement. According to defense analysts, rumors have circulated online that Ukrainian special forces, partisans, drones, or long-range rockets were responsible, which would represent a serious security failure at an important Russian air base some 200 kilometers (124 miles) behind the front lines.

Oksana Markarova, Ukraine’s ambassador to the United States, told Bloomberg’s Washington office that while she couldn’t go into detail about what happened in Crimea, Russians “should not feel safe anywhere in Ukraine as invaders.”

She stated that it is difficult for Ukrainians in occupied Crimea to resist after “eight years of pressure,” but there are more signs of activity as “people see we are more capable of actually freeing some territories.”

The destruction of combat aircraft at Saky follows an unexplained July attack on Russia’s Black Sea Fleet headquarters in Crimea. According to intelligence officials, the suspected Ukrainian assault will change Moscow’s perception of Crimea as a safe haven for its invasion forces, especially if more strikes follow.

This could force Russia’s already stretched military to divert resources to protect previously thought-secure areas, just as Ukraine’s army presses on toward Kherson, the southern city seized by Moscow’s troops early in the war. A massive road and rail bridge connecting Crimea to Russia across the Kerch Strait is an obvious target for Ukraine’s military to cut supply lines to an important staging point for Kremlin forces.

On Thursday, Belarus dismissed reports of explosions near the Ukrainian border at an airbase used by Russia’s military during the invasion, according to opposition activists. A fire broke out at the Zyabrovka base late Wednesday and was quickly extinguished, according to the state-run Belta news service, citing a Belarus Defense Ministry statement.

Satellite imagery of the Crimea base revealed that buildings and aircraft had suffered extensive damage. According to Oryxspioenkop, a Dutch open-source intelligence defense analysis website, Russian losses include at least four Su-30SM multirole aircraft and six Su-24M/MR strike and tactical reconnaissance aircraft.

According to Ben Barry, a senior fellow for land warfare at the London-based International Institute for Strategic Studies, the Crimea explosions are most likely the result of sabotage operations carried out by Ukrainian special forces or partisans.

“If Russia wants to protect their airbases, it can only come at the expense of troops for the Ukrainian attack,” he said. “It may reduce the number of Russian forces on the front lines, but we don’t know whether that will be decisive.”

Ukrainian officials have not directly claimed responsibility, but have suggested in social media posts that their forces were responsible for the explosions. The blasts, according to a top Zelenskiy aide, were “just the beginning” of efforts to reclaim Crimea after Russian President Vladimir Putin annexed the Black Sea peninsula in 2014.

This raises the stakes for Putin, who has repeatedly declared Crimea to be an inalienable part of Russia, a claim rejected by the international community, which considers it Ukrainian territory.

In an interview with the BBC, UK Defense Secretary Ben Wallace dismissed Russia’s explanations for the explosions, saying it was clear the explosions were not caused by someone dropping a cigarette. It would be legitimate to target Russian forces in Crimea because “Ukraine, under United Nations articles, is perfectly entitled to defend its territory and take what action it needs to against an invading force,” he said.