JERUSALEM - The Israeli Navy will soon reinstate one of its submarines that had been secretly upgraded as part of wider efforts to bolster Israel's long-range strategic capabilities.

The renovation of the Dolphin-class submarine, one of the three that Israel currently deploys, included a comprehensive structural overhaul, The Jerusalem Post reported Monday.

Israel has invested near $27 million?into the project. The renovation work at the navy's northern Haifa shipyard, launched two years ago, was carried out clandestinely so as not to alert Israel's enemies to the fact that one of its submarines was temporarily decommissioned, according to the report.

"Every vessel that comes into the shipyard for maintenance comes out with improved capabilities ... There are a select number of countries around the world which can independently renovate a submarine," Col. Eli Shouach, commander of the shipyard, told the newspaper.

The officer in charge of the project said it included dismantling the submersible's engine, valves, pipes and sonar for cleaning, while cracks in the hull were welded.

Navy officials would not say when the submarine, which entered service in 1999, is scheduled to hit the water, but the upgrade will enable it to remain operational for at least another 18 years.

The disclosure came ahead of the graduation ceremony of the navy's 100th Submariner Course, a prestigious program that trains officers for command posts in the Flotilla 7 submarine fleet.

Israel's diesel-powered Dolphin submarines, all manufactured at Kiel dockyards in Germany and partially underwritten by the German government, are believed to be capable of firing cruise missiles armed with nuclear warheads. Another two vessels, still undergoing construction, will be delivered to Israel in the coming years.

While Israel maintains an official policy known as "nuclear ambiguity," according to which the Jewish state will not be the first or last nation to introduce nuclear weapons in the Middle East, its submarines are designated as a deterrence weapon that affords a second-strike capability.