Iran's fleet of twenty submarines poses a dangerous and unpredictable force in the Persian Gulf region.

The United States - with its Fifth Fleet is based in Bahrain and carrier groups frequently deployed in the region - has not revealed what it knows about Iran's capabilities.

Military analysts believe the U.S. has been using aircraft, satellites, subs, and underwater sensors to monitor the activities of Iranian subs at sea.

Doing so requires entering Iranian territorial waters, but the Iranians would be unlikely to admit US warships entered their waters with impunity publicly.

Recent US war games which included achieving naval dominance, air superiority, and a coastal foothold in a fictional nation closely resembling Iran were said to include an anti-submarine warfare component.

But the efficacy of Iran's submarine fleet remains unknown due to the untested nature of most of Tehran's designs.

Russia agreed to stop selling its Kilo-class subs to Iran in 1996, forcing Tehran to develop homegrown craft. Tehran has three such vessels.

After ten years of trial and error they produced the 100 ton Qadir class vessels in 2005. Iran currently has 12 of these small diesel electric subs. The Qadir-class vessels are squarely between the old midget submarines and the Russian Kilos.

Analysts say look very similar to the Italian made Cosmos SX-506B submarines that Columbia received in the 1980s. The 100-ton SX-506Bs are only large enough to carry commandos and mines. News photos also show what may be two torpedo tubes, as well.

Russia's Cosmos exported a number of larger SX-756 vessels to Pakistan in the 1990s, which may be the design basis for the Qadir. The North Korean Sang-O class submarine closely approximates the Qadir type.

In 2007, North Korea gave Iran four of its Yugo-type midget submarines. These elderly Yugos are 90-ton, 65-foot craft.

Iran is also believed to possess five larger Nahang class subs. A about 500-tons it is the same size as and closely resembled the old German Type-206 class from the 1960's, which was developed for operations in the confined shallows of the Baltic.

The Type 206’s size enabled it to carry eight torpedo tubes with no reloads, but the Iranian version has been little seen and Western intelligence officials believe it is a failed design.

Tehran is now working on the third indigenous Iranian design. Laid down in 2008, the Qaaem is a 1000-ton boat and should be large enough to handle a full set of torpedo tubes along with a reload. They could be the possible replacement for Iran’s Russian-made Kilos.

Iran's Kilo 877/636 type diesel-electric submarines pose the most significant threat in Tehran's submarine fleet. The 2300 ton Kilos are long range subs capable of operating throughout the Indian Ocean.

They have six 533mm (21 inch) torpedo tubes and carry 18 torpedoes, or 24 mines. The Kilo is regarded as a formidable foe and stay at sea for up to 45 days. Iranian Kilo crews have more than a decade of experience, but the craft are more than half-way past their 30-year lifespan.

The real question is whether Iran has found ways to overcome their limited experience and technological know-how with warship construction in general, and submarine construction in particular.

The Qadir boats are reported to be troublesome for their crews and unsafe, which some analysts say may be indicative of Iranian submarine design overall.

How they would perform against the naval power and prowess of the United States, or Israel's German-manufactured Dolphin-class submarines, will remain unknown unless a shooting war erupts.