The Israeli navy is getting ready to expand its fleet of German-built Dolphin-class submarines that are widely believed to give it the only seaborne nuclear missile capability in the Middle East.

Three early-model Dolphins are already in service and reportedly range as far as the Indian Ocean south of Iran. But the navy's moving closer to deploying two more of the 1,720-ton, diesel-electric boats built by Howaldtswerke-Deutsche Werft in the Baltic port of Kiel. HWD is a unit of ThyssenKrupp Marine Systems.

The fourth Dolphin, christened the Tanin, was handed over to the Israeli navy by HDW in May 2012 and is due to become operational within the next few months following sea tests and evaluation.

The fifth boat, the Rahav, was launched in Kiel April 29 and is expected to arrive in Israel's northern port of Haifa, the submarine fleet's headquarters and main base, around mid-2014.

A contract for a sixth Dolphin, the most advanced of the series, was signed with the German government in May 2012 after differences over payment.

German Chancellor Angela Merkel also imposed a series of political conditions on Israeli Prime Minister Binyamin Netanyahu, including unblocking $100 million a month in customs duties imposed on the Palestinian Authority and other funds blocked by Israel.

The sixth Dolphin is scheduled to reach Israel in 2017.

Little information on the Dolphin operations is ever released, though it is general understood that with the current three boats operational, one is on patrol in the Red Sea or Indian Ocean, covering Iran and its gunrunning routes to Hamas in the Gaza Strip and Hezbollah in Lebanon.

One is at Haifa on refit, while the third is cruising the Mediterranean.

After the Israelis supposedly knocked out an arms depot outside the Syrian port and naval base at Latakia July 5, where the regime was said to be storing ship-killing, Russian-supplied P-800 Yakhont missiles, there were reports -- never substantiated -- that a Dolphin in the Mediterranean had unleashed a broadside of land-attack missiles on the site.

The Dolphins carry conventional versions of the Popeye Turbo cruise missile for that kind of mission. These are manufactured by Israel's Rafael Advanced Defense Systems.

The navy adapted the original air-launched version of the Popeye for the Dolphin force. The U.S. Navy tracked a secret Israeli submarine-launched Popeye test in the Indian Ocean in 2002 in which the missile hit a target at a range of nearly 950 miles.

The Dolphins, based on the design of HDW's U-209 class sub, have a range of about 2,700 miles, although this has been likely extended in the three advanced models Israel's now getting.

Expanding the Jewish state's submarine force from three units to six is no trivial matter since it will involve finding and training men for the Dolphins, which usually carry 35-man crews.

These systems will form the navy's strategic spearhead that will add immense firepower to Israel's nuclear triad of air-, land- and sea-based weapons, which in the event of war with Iran over its contentious nuclear program would play a vital role in taking out nuclear facilities or other strategic targets.

Manning the new Dolphins, and having backup crews for rotations, will have to be implemented without weakening the quality of existing crews.

The Israeli military's Bamachaneh magazine reports that the number of personnel selected for submarine warfare has grown by 30 percent in recent recruitment intakes.

According to published reports in Israel, that's a significant shift in a country where the arm and the air force traditionally have been given precedence when it comes to top-quality recruits.

Israel's Arutz Sheva news outlet reported that more officers are being trained for submarine posts and the number of cadets who will be trained for submarine command has been rising by 35 percent.

The head of the navy's high school outreach program, identified only as Maj. Yisrael, said the project began in 2012 as the new subs were still being built in Kiel. He expects about 30 percent of the young sailors who attend a five-day introductory program at the Naval Instruction Base at Haifa this year will reach the navy's training course phase after enlisting.

The major told one group of 11th-graders: "To serve in submarines is unique ... . This is all-important work but it won't be publicized and submarine crew members can't tell anyone what they do."