Sky News has been shown the graves of Iranians killed while fighting in Syria.

We were invited to a corner of Tehran's national war cemetery where the soldiers are buried - somewhere foreign crews are never normally allowed.

Above new graves hang large posters celebrating the sacrifices the dead have made in a foreign land. Beneath them mourners pay their respects.

On his mobile phone, one man showed us pictures of a fallen friend he said had died in a holy war.

"I don't think of it as a different country because defending Islam has no borders," he said.


"They have gone there to defend Islam."

Another man was mourning an old school friend killed fighting near Aleppo.

Image: A mourner pays his respects at Iran's national war cemetery

"First I was really upset but inside my heart I'm very happy that he has become a martyr. Not everyone is lucky enough to become a martyr," he said.

The fact we were allowed to film here shows a new willingness on the part of Iran to talk about its fighting in Syria.

For a long time Iran's war in Syria was secret. It concealed its military activity there.

We saw only glimpses, the odd mobile phone photograph of Iranians being briefed alongside Syrian soldiers, for instance.

Even now, officially, Iran says it only has "military advisers" in Syria.

In reality, hundreds, if not thousands, of Iranian Revolutionary Guard Corps (IRGC) fighters - including Quds special forces - are known to be operating in Syria.

Several hundred Iranians, including a number of senior commanders, are known to have died fighting alongside Shia militia forces from Hezbollah and Assad regime ground forces.

Image: Mourners grieve a member of Iran's Revolutionary Guard killed in Aleppo

Protecting Syria's regime is strategically important for Iran. Syria is an important ally and conduit for arms supplies to Hezbollah in southern Lebanon.

But Iran insists its primary objective in Syria is fighting IS before it can become a threat within its own borders.

We heard that justification throughout our time filming in Iran.

At Friday prayers IRGC commander Naser Shabani, who had just returned from the front in Syria, told the faithful that IS, also known as ISIS, had to be dealt with to protect them at home.

We heard it on the streets and also in Iran's parliament, where I asked MP Mohammad Alipour, spokesman for the parliamentary economics committee, how long Iran could afford to sustain its military involvement in Syria.

"Any sort of support has some sort of economic cost, but we believe if we do not make that sacrifice and fight against IS in Syria beyond our borders then we'd have to do it inside them, which will be a much bigger and worse burden," he said.

How Well Do You Know Iran?

Analysts here say Iran has several reasons to fight in Syria. Shashank Joshi from the Royal United Services Institute says Iran may be deliberately signalling a continuing commitment to remain involved.

"The very fact that Iran is talking more openly about its role in Syria shows they are ready to defend their position and their legitimacy of their support for Assad in a way that is very unusual and perhaps signifies a degree or an effort to get public international support and portray themselves as being against ISIS," he said.

Iran is also determined to defend Shia shrines in Syria from IS desecration. We were told several soldiers buried in the cemetery had died in operations protecting those shrines.

Whatever the motivation, Iran seems as committed as ever to fighting in Syria, alongside an array of other outside powers on either side of the proxy war there.

Cemeteries across the region will continue filling with the dead.