Breaking news: Ukraine claims first Sub Sea Baby underwater drone strike on Russian Kilo-class submarine.

Ukraine’s Security Service says it disabled a Russian Project 636.3 Improved Kilo-class submarine at Novorossiysk using an underwater drone called Sub Sea Baby. If confirmed, the strike would mark a major shift in naval warfare by showing that even protected submarine bases are vulnerable to low-cost unmanned attacks.

Ukraine’s Security Service, known as the SBU, claimed on December 15, 2025, that it successfully struck a Russian Improved Kilo-class submarine inside the port of Novorossiysk using an underwater unmanned system, a platform the agency refers to as Sub Sea Baby. According to the SBU, the operation was conducted jointly with the Naval Forces of Ukraine through military counterintelligence channels and caused critical damage that rendered the submarine non-operational, a claim that cannot yet be independently verified through imagery or official Russian statements.
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Ukraine’s Security Service video showing a Russian Project 636.3 Varshavyanka Kilo class submarine, which the SBU claims was critically damaged in Novorossiysk by an underwater Sub Sea Baby drone during a joint operation with the Ukrainian Navy (Picture source: Ukraine's Security Service).

If the claim is confirmed, the tactical meaning is hard to overstate because submarines are among the most difficult naval assets to target, especially inside a defended harbor. A diesel electric boat can disappear at sea for weeks, and even in port, it is typically sheltered by layered security that may include controlled access zones, patrol craft, surveillance, and physical barriers intended to stop unmanned surface craft and divers before they reach the hull. Past satellite-based reporting has shown Russia experimenting with barrier-based defenses at Novorossiysk as Ukraine’s maritime drone campaign intensified, an acknowledgement that the base is not a sanctuary even on Russian soil.

The SBU statement adds a detail that deserves careful attention: it differentiates between the surface “Sea Baby” unmanned surface vessels that have become familiar since 2023 and an underwater “Sub Sea Baby” used for this strike. The SBU has publicly demonstrated that the Sea Baby family has evolved from a way explosive craft into a modular system with longer range and heavier payload. Upgraded Sea Baby variants are reported to have a range exceeding 1,500 kilometers and payload capacity up to 2,000 kilograms, and some versions are designed to carry remotely controlled weapons rather than acting solely as kamikaze craft. Other reported improvements include AI-assisted targeting, stabilized weapon stations, rocket launchers, and layered self-destruct measures intended to prevent capture and exploitation.

What the SBU has not published, at least in its initial announcement, is a detailed technical profile of the “Sub Sea Baby.” That absence matters because the distinction between a surface drone and an underwater system is decisive in determining whether harbor barriers and surface patrols remain effective. An underwater approach can exploit blind zones in port security, reduce exposure to defensive fire, and reach a submarine at its most vulnerable area below the waterline. In operational terms, a successful underwater strike implies reliable navigation and control in a cluttered harbor environment, as well as a warhead and fuzing concept capable of damaging propulsion, pressure hull adjacent systems, or other mission-critical components while the boat is alongside.

The target described by the SBU, Project 636.3, is Russia’s Improved Kilo class, designed for quiet operations in contested littoral waters. Open technical data places the 636.3 at approximately 74 meters in length with submerged displacement close to 4,000 tons, diesel electric propulsion, and a crew of around 50 personnel. Its primary armament consists of six 533 millimeter torpedo tubes capable of firing heavyweight torpedoes, laying naval mines, and launching Kalibr cruise missiles from submerged positions. In missile-capable configurations, the class can deliver short interval salvos, giving it a credible land attack and anti-ship strike role in addition to traditional sea denial missions.

The SBU claims the submarine in Novorossiysk carried four Kalibr launchers, directly linking the strike to Russia’s long-range missile campaign against Ukrainian territory. Kalibr missiles are assessed to have a range of roughly 1,500 to 2,500 kilometers, depending on variant, which is why each missile-capable submarine is treated as a strategic asset rather than a purely tactical platform. Even without sinking the boat, damage severe enough to require major repairs can remove it from the operational cycle for months or longer. International sanctions further complicate access to spare parts, shipyard capacity, and specialized components, strengthening the operational impact.

For Ukraine, the achievement, if verified, would underline a harsh reality for the Russian Navy: acoustic stealth does not guarantee safety when an adversary can deliver precision explosives using low-cost unmanned systems. Russian naval units have already been compelled to disperse from Sevastopol under pressure from Ukrainian maritime strikes, with Novorossiysk emerging as a key alternative hub. An underwater strike at that port would indicate that Russia’s defensive challenge is expanding from sea control to base security, forcing costly investments in sensors, patrols, barriers, and counter-sabotage measures that demand constant manpower and vigilance.

For Russia, a disabled Improved Kilo represents both an immediate reduction in Kalibr launch capacity and a psychological blow to confidence in port safety. If Ukraine has indeed fielded an operational underwater derivative of the Sea Baby family, the Black Sea conflict is entering a new phase in which even heavily protected infrastructure and concealed assets can be threatened at their moorings, further shifting the balance between expensive legacy platforms and rapidly evolving unmanned strike systems.