Israeli defense officials blocked an effort by Ukraine to obtain NSO Group’sPegasus surveillance software over fears doing so could inflame tensions with Russia.

Sources cited by The Guardian and The Washington Post claim Ukraine has been trying to get its hands on the coveted hacking tools since 2019. They aren’t alone. Intelligence agencies worldwide jockey over access to Pegasus, which is viewed as one of the most advanced zero-click hacking tools on the market. Once infected, Pegasus software allows the end-user to surveil the target’s photos, documents, and even encrypted messages without the target ever knowing.

Getting access to the software can be tricky, though. According to sources cited by The Guardian, Israel’s ministry of defense must first grant permission to NSO before the company can market its software to clients from other countries.

Researchers at The Citizen Lab have identified at least 45 counties where Pegasus has been deployed in the past. Many of these countries have usedPegasus to target journalists, human rights activists, political dissidents, and even children with alarming frequency.

Sources speaking with The Guardian said Israeli officials worried Ukrainian efforts to track or monitor Russian phone numbers with the software would be seen as an act of aggression against Russia. The U.S., usually tied to the hip with Israel on geopolitical concerns, was reportedly in favor of granting Ukraine access to the software. Nearby neighbor Estonia, meanwhile, was reportedly given access to Pegasus back in 2019 but had their licenses revoked last year under unclear circumstances.

NSO did not immediately respond to Gizmodo’s request for comment.

While the U.S. and many European countries have rushed to slap severe sanctions on Russia and shower Ukraine with resources and militaryequipment, Israel acted more conservatively. That reluctance to upset Russia gained the ire of Ukrainian president Volodymyr Zelensky, who released a video blasting the Israeli parliament for refusing to “save the lives of Ukrainian Jews.”

“What is this, indifference, calculations, mediation without choosing sides?” Zelensky said in a translated video. “You can mediate between countries but not between the bad and the good and Israel knows well the Iron Dome, your missile defense system is the best.”

Recent reporting from the New York Times and others has detailed NSO’s close relationship with the Israeli government. In some cases, according to the Times, Israel has seemingly dangled access to NSO’s spyware as an incentive to convince countries like Mexico and Panama to take more favorable policy positions towards Israel. In Mexico’s case, security agents then allegedlysubsequently used Pegasus to suppress domestic dissent.