Iran has more than half a million active service personnel (Picture: Iranian Presidency/DPA via PA).
Iran has warned it will respond "when necessary" to the seizure of a tanker off the coast of Gibraltar last week and that the incident will not go "unanswered".
The Iran military's chief of staff said the capture of the Grace 1 was based on "fabricated excuses".
Meanwhile, tensions between the United States and Iran continued recently when an American drone was shot down.
Tehran says the drone was shot down over Iranian airspace, while US officials say the incident took place over the Strait of Hormuz, in international airspace.
Meanwhile, Donald Trump’s decision to withdraw the United States from an international nuclear deal with Iran earlier this year reignited tensions with the regional military power.
Iran’s influence in the Middle East has grown through conflicts in Lebanon, Iraq, Syria and Yemen, fuelling tensions its longstanding rival Saudi Arabia.
In February, President Trump said he will be keeping troops in Iraq so he can "watch Iran", a country he described as "a real problem".
Here are some points on Iran, one of the Persian Gulf's major powers:
1) Iran’s military consists of two parallel organisations; the regular armed forces and the Islamic Revolutionary Guards Corps (IRGC).
2) Iran's active service personnel number 523,000 with 350,000 in reserve.
3) The Iranian defence budget for 2018 was just over £15.5bn. A fall of more than £1.1bn on the previous year.
4) Iran spends around 4.5% of GDP on its defence.
5) Iran’s armed forces struggle with ageing hardware but ingenuity and upgrades have helped the air force maintain a fleet including US-made F-14 Tomcats and F-4 Phantom jets and last year Iran unveiled what it said was a fourth generation fighter.
6) Iran has developed ballistic missiles with a range of up to 2,000 kilometres (1,242 miles), such as the Sejjil series, but argues they are defensive and not designed to carry nuclear warheads.
7) Iran's navy has been showcasing its Ghadir-class mini-submarines which can launch cruise missiles. The boats are believed to carry a crew of nine and weighs less than 150 metric tons.
8) In 2011 Iran took control and captured an unmanned US Lockheed Martin RQ-170 Sentinel while in flight and used the technology to create its own drone.
Sources: International Institute for Strategic Studies, Central Intelligence Agency.