Ukraine’s security service SBU said on Tuesday it had uncovered a network of agents working for Russian military intelligence trying to find out where Ukraine was keeping its Western-supplied F-16 fighter jets.

Kiev received its first F-16 fighter jets in August after months of pleading with its Western allies to send the planes to help intercept Russian missiles. Their location is a closely guarded secret.

The SBU said it had neutralized “a large agent network” of 12 agents and their informants who were “trying to identify the locations of secret military airfields where, according to the Russians, F-16 aircraft may be based.”

The agents are also said to have conducted reconnaissance operations at the firing positions of Ukrainian air defense systems and tried to find companies producing electronic warfare equipment to counter drones.

Ukraine’s SBU said the group’s leader was based in the Dnipropetrovsk region and passed information to a Russian agent, while agents were operating in five regions.

The SBU released images of four arrested men with blurred faces. If convicted, they face eight years to life in prison.