How strong is Iran's military?

Iran has vowed to retaliate after its most powerful military commander was killed by a US drone strike at Baghdad airport.

"Severe revenge awaits" those behind the attack on General Qasem Soleimani, said Iran's Supreme Leader.

So what do we know about Iran's military capabilities?

How big is Iran's army?
There are an estimated 523,000 active Iranian personnel in a range of military roles, according to the International Institute for Strategic Studies, a UK-based think tank.

This includes 350,000 in the regular army, and at least 150,000 in the Islamic Revolutionary Guards Corps (IRGC).

There are a further 20,000 service personnel in the IRGC's naval forces. This group operates a number of armed patrol boats in the Strait of Hormuz, the site of several confrontations involving foreign-flagged tankers in 2019.

The IRGC also controls the Basij unit, a volunteer force which has helped suppress internal dissent. This unit can potentially mobilise many hundreds of thousands of personnel.

The IRGC was set up 40 years ago to defend the Islamic system in Iran and has become a major military, political and economic force in its own right.

Despite having fewer troops than the regular army, it is considered the most authoritative military force in Iran.

What about operations abroad?
The Quds Force, which was led by General Soleimani, conducts secret operations abroad for the IRGC and reports directly to the Supreme Leader of Iran, Ayatollah Ali Khamenei. It's believed to be about 5,000 strong.

The unit has been deployed to Syria, where it advised military elements loyal to Syrian President Bashar al-Assad and armed Shia militias fighting with them. In Iraq, it has supported a Shia-dominated paramilitary force which assisted in the defeat of the Islamic State group.

However, the US says the Quds force has a wider role by providing funding, training, weapons and equipment to organisations that Washington has designated as terrorist groups in the Middle East. These include Lebanon's Hezbollah movement and the Palestinian Islamic Jihad.