Israel on Tuesday took delivery of its fifth German-built submarine, an advanced Dolphin-class vessel said to be capable of remaining submerged for up to a week.
Speaking at an official welcome ceremony at the northern port city of Haifa, Prime Minister Benjamin Netanyahu said the undersea fleet allows Israel "to deter enemies who seek to destroy us."
"They should know that Israel can strike very hard indeed at anyone who tries to harm it," he said.
The new arrival is named "Rahav" after a biblical sea monster.
"Rahav will take an active part in defending the state of Israel and its territorial waters, operating deeper, further, and for longer from the very depths -- with a watchful eye," President Reuven Rivlin said at the ceremony.
Foreign military sources say the Dolphins can be equipped with missiles armed with nuclear warheads.
They say Israel has between 100 and 200 warheads and missiles capable of delivering them.
Israel is the Middle East's sole if undeclared nuclear power, refusing to confirm or deny it has such weapons.
Its five German-made submarines will be used to protect its shores and carry out spying missions against its arch-foe Iran, Israeli media say.
Netanyahu tried in vain to block a July deal with world powers on scaling down Iran's nuclear programme in exchange for sanctions relief, arguing it would not stop Tehran from developing an atomic weapon.
The incoming head of Israel's Mossad spy agency said last week that the Islamic republic and its nuclear ambitions constitute "the principle challenge" for his organisation.
A sixth submarine is to be delivered in two to three years although defence analyst Yossi Melman, writing in Maariv newspaper, has said it is likely to be cancelled for budgetary reasons.
The current model costs about 500 million euros ($540 million) to build, Israeli media say. Berlin is paying one third of the cost itself.