India has taken a significant step in strengthening its nuclear deterrence with the successful test launch of the K-4 submarine-launched ballistic missile (SLBM) on December 23, 2025. Multiple credible sources, including The Times of India, confirmed that the missile was test-fired from the 6,000-tonne nuclear-powered ballistic missile submarine (SSBN) INS Arighat, operated by India’s tri-service Strategic Forces Command (SFC).
The Ministry of Defence (MoD) has, as is often the case, not issued any official statement regarding the test. Historically, the MoD has remained silent on SLBM tests, opting for discreet reporting through indirect channels. A source quoted by The Times of India highlighted the meticulous evaluation process that follows such launches: “A comprehensive analysis will determine whether Tuesday’s test actually met all laid-down technical parameters and mission objectives or revealed some shortcomings. It usually takes several tests for ballistic missiles, especially those launched from submarines, to achieve full operational status.”
This recent test follows the earlier November 2024 launch of the K-4 from INS Arighat, marking the first launch of the missile from an operational submarine. That test, conducted over nearly the missile’s full range of 3,500 km, was a significant milestone for India’s nascent undersea deterrence capability. INS Arighat, commissioned in August 2024 as the Strategic Forces Command’s second SSBN, complements the lead vessel, INS Arihant, in India’s growing nuclear triad.
Submarine-launched ballistic missiles differ from their land-based counterparts in fundamental ways. Unlike land-based systems such as the Agni series, SLBMs must endure the high pressure of underwater launch, sometimes from depths of over 10 meters. This requirement necessitates a sturdier, heavier construction. The K-4, for instance, is a 10-12 meter long, 1.3-meter diameter solid-fueled missile weighing 17–20 tonnes and capable of carrying a two-tonne nuclear warhead. Its construction reflects the rigorous engineering standards necessary to withstand underwater launch pressures while maintaining accuracy and reliability.
India’s first operational SLBM, the K-15 (also known as B-05), has a relatively limited 750-km range but was pivotal in laying the foundation for sea-based deterrence. The K-15 can carry a warhead of up to one tonne, guided primarily by inertial navigation, with terminal homing enhancing accuracy. While the K-15 extended India’s nuclear triad capability, its shorter range required submarines to operate closer to potential adversary coastlines, limiting survivability.
The K-4 SLBM, by contrast, provides a significantly extended strike range of 3,500 km, allowing SSBNs to patrol in more secure waters while retaining the ability to conduct retaliatory strikes. The missile is designed to maneuver during re-entry to evade missile defense systems, achieving a 100-meter Circular Error Probability (CEP) to maintain accuracy over long distances.
The K-4 is a product of India’s indigenous defense research and development ecosystem, spearheaded by the Defence Research and Development Organisation (DRDO). Critical components of the missile were developed at the Pune-headquartered Armament and Combat Engineering (ACE) cluster:
High Energy Materials Research Laboratory (HEMRL), Pune – developed solid propellants and motor systems.
Research & Development Establishment (Engineers), Pune (R&DE Engrs) – responsible for the missile’s launch system.
Advanced Centre for Energetic Materials (ACEM), Nashik – processes composite propellants for the missile.
The Naval Systems Group of DRDO additionally developed the underwater launch system for the K-4, integrating it seamlessly with the SSBN platform. The missile’s robust design, solid-fueled propulsion, and advanced warhead capabilities reflect years of developmental work, building upon lessons learned from earlier systems like the K-15.
India’s SSBN fleet is central to achieving a credible second-strike capability. INS Arihant and INS Arighat currently carry either four K-4 missiles or twelve K-15 missiles, providing flexibility in operational deployment. However, the Strategic Forces Command is already planning the next generation of SLBMs and SSBNs to expand both range and survivability:
K-5: A 5,000-km-range SLBM under development. At 12 meters in length, it is designed for patrols farther from adversary territories, enhancing survivability.
K-6: A 6,000-km-range missile, reportedly capable of carrying a three-tonne warhead. The missile, 12 meters long with a 2-meter diameter, is also claimed to feature hypersonic speeds (up to Mach 7.5) and MIRV (Multiple Independently Targeted Re-entry Vehicle) capability, enabling it to deliver multiple warheads against separate targets.
Future Arihant-class Stretch SSBNs, with 7,000-tonne displacement and 125-meter length, will carry eight K-4 missiles or twenty-four K-15s, or a combination of both. A further clean-sheet S-5-class SSBN is planned, featuring 13,300-tonne displacement and twelve K-6 missiles, powered by a 190-MW reactor designed by the Bhabha Atomic Research Centre.
The operationalization of the K-4 SLBM will represent a landmark in India’s undersea deterrent capability. While the K-15 provided a limited initial strike option, the K-4 allows for secure, long-range patrols from within India’s maritime zone, enhancing the survivability of India’s nuclear arsenal.
A former head of the Strategic Forces Command has noted that India’s SSBNs would ultimately operate from “secure havens” close to the country’s coasts, leveraging the extended range of newer SLBMs to maintain credible second-strike capability. This strategy mirrors global best practices, where a secure undersea deterrent forms a key leg of a nation’s nuclear triad, ensuring that even if land-based assets are neutralized, retaliatory capability remains intact.
Experts suggest that once K-4 user trials are fully validated, India’s undersea deterrent will gain operational credibility. The long-range K-4 SLBM addresses one of the key limitations of India’s first-generation SLBM, enabling SSBNs to patrol in safer, undisclosed waters without approaching potential adversary coastlines.
The induction of the K-4 and the commissioning of INS Arighat strengthen India’s nuclear triad, consisting of land-based missiles (Agni series), air-launched weapons, and sea-based SLBMs. The triad ensures a survivable second-strike capability, critical to maintaining strategic stability in South Asia.
While the K-6 missile is still under development and its claimed hypersonic MIRV capability remains unverified, the trajectory of India’s SLBM program indicates a clear intention to match emerging global standards for sea-based nuclear deterrence. Full operationalization of K-6 would position India among a small group of nations capable of deploying extremely long-range, multiple-warhead SLBMs from secure maritime patrol zones.
The K-4 launch also signals India’s commitment to modernizing its nuclear forces in response to evolving regional and global strategic environments. With China and Pakistan both actively expanding their nuclear arsenals, a credible sea-based deterrent becomes central to India’s security calculus.
The recent K-4 test marks another step toward the maturity of India’s undersea nuclear capability. While official confirmation from the MoD remains absent, operational testing of the K-4 from INS Arighat underscores India’s ability to indigenously design, develop, and deploy advanced SLBMs.
In the near term, the focus will likely be on completing user trials and validating the missile’s operational reliability under combat-like conditions. Following successful K-4 deployment, attention will turn to the K-5 and K-6 missiles, along with the commissioning of the Arihant-class Stretch and S-5-class SSBNs. These steps will collectively provide India with a highly credible, survivable sea-based deterrent capable of securing national security interests well into the next decade.
As India continues to build and test advanced SLBMs, the Strategic Forces Command will maintain its emphasis on secrecy, strategic ambiguity, and operational readiness—hallmarks of a robust nuclear deterrent strategy. The December 23 K-4 launch, though quietly executed, represents a tangible signal of India’s growing undersea nuclear capability and its intent to maintain a credible and credible second-strike posture.
