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India has conducted a user trial of the K-4 submarine-launched ballistic missile from INS Arihant in the Bay of Bengal on 23 December 2025. This test, achieving a range of approximately 3,500 km, represents a key advancement in the sea-based component of India's nuclear triad.

Shiv Aroor a renowned defence journalist had confirmed the SLBM launch this on a social media handle. Open-source reports from several defence observers and social media handles have widely circulated details, though the Ministry of Defence has yet to issue an official confirmation as of 24 December.
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The K-4 missile, developed by the Defence Research and Development Organisation, serves as an intermediate-range SLBM designed for Arihant-class submarines. It features a two-stage solid-fuel configuration, measuring around 12 metres in length and weighing nearly 17 tons, with capabilities for cold launch from underwater. Advanced inertial navigation augmented by GPS/NavIC ensures high accuracy, with a circular error probable below 10 metres, while manoeuvring aids evade missile defences.

This trial extends India's sea-based strike envelope far beyond the K-15 Sagarika's 750 km range, enabling SSBNs to maintain survivable patrols without approaching enemy coasts.

Such operations underpin India's "No First Use" doctrine and credible minimum deterrence by guaranteeing a robust second-strike option. Regular user training launches from operational submarines signal maturation from developmental tests to repeatable deterrence missions.

INS Arihant, India's lead Arihant-class SSBN built under the Advanced Technology Vessel programme at Visakhapatnam, accommodates four vertical launch tubes for either multiple K-15s or fewer K-4s. The Bay of Bengal remains the primary test corridor, supported by NOTAMs and exclusion zones observed in mid-December, aligning with patterns for long-range trials. Recent airspace and maritime notices had foreshadowed activity in the 21-30 December window.

India's SSBN fleet expansion bolsters continuous at-sea deterrence. INS Arighaat, the second Arihant-class boat, commissioned in August 2024, already integrates K-4 capabilities. INS Aridhaman, the third vessel with an extended hull for eight launch tubes, nears commissioning in early 2026 after extensive trials. Navy Chief statements in December highlight this as enhancing patrol availability amid regional tensions.

Regional dynamics amplify the test's significance. Chatham House noted escalation risks from K-4 activities, with Chinese maritime surveillance potentially tracking Indian Ocean operations. The 3,500 km reach targets strategic depths while preserving SSBN stealth in oceanic bastions. Future K-5 and K-6 missiles for S5-class submarines promise further evolution into the 2030s.

If officially acknowledged, focus may shift to whether this was routine training, configuration validation, or readiness for fleet growth. Such milestones affirm the sea leg's transition to operational maturity, fortifying India's nuclear posture against multi-domain threats. Defence watchers anticipate detailed analysis of flight profile and performance to refine integration.

IDN (With Social Media Inputs)