It's now been six months since Islamic State forces swept into northern Iraq, taking Mosul, the country's second largest city and routing four brigades of the Iraqi army before stopping short just north of Baghdad. Days later, with reports of massacres against Shias and minority groups, Ayatollah Ali al-Sistani, the most important Shia cleric in Iraq, issued a fatwa, calling on all "able-bodied men" to help the Iraqi Security Forces (ISF) combat the Islamic State. The call to arms was enthusiastically answered, with tens of thousands joining Iraqi Shia volunteer forces, some heading north to help defend Baghdad, others headed south to protect the Shia shrines in Karbala and Najaf.

With the Iraqi army demoralised after the collapse of their troops in Mosul and now busy preparing a force for a possible spring counteroffensive, the government has been forced to rely on the volunteers to do much of the fighting against the Islamic State (IS). With this responsibility, these paramilitary groups have grown in power and legitimacy. After the recent elections Mohammed al-Ghoban, a member of the Badr Organisation - a political bloc that also has a military wing regarded as the largest of the volunteer brigades - was installed as the new interior minister.

It is these paramilitaries who are largely being touted as responsible for a string of successes against IS on the ground over the past four months, forcing them back from the outskirts of Baghdad and pushing them further north. Despite their successes, allegations of human rights abuses against Sunni communities have been leveled in their direction and their purported assistance from Iran has raised many eyebrows in the West.

Faleeh Khazaali, a KSS commander and parliamentarian, lost an eye in a firefight against Jabhat al Nusra in Syria, where he had been protecting Shia shrines. Images via Henry Langston.

We spent time with the Kata'ib Sayyid al-Shuhada (KSS) brigade, which was involved in the resistance against IS early on, helping to secure the northern outskirts of Baghdad and the city of Samarra some 50 miles north of the city.

Like many of their fighters, Faleeh Khazaali, a commander of the KSS and an Iraqi parliamentarian from Basra, had been previously battling to protect Shia shrines in Syria, but after losing an eye in a firefight against Jabhat al Nusra, he was forced to return to Iraq. We joined him in a convoy north to visit some of his forces on the front lines around Samarra, stopping first on the outskirts of Baghdad, also known as the Baghdad Belt, to show some of the areas in which the KSS first engaged IS.

Their first mission was to secure a number of bridges along the river Tigris. It fell to them, Khazaali said, "to safeguard the citizens of Baghdad, without us, IS would've slaughtered the men and enslaved our women."

"The Shuhada paid in blood to achieve that," he added.

The KSS travels in an armored vehicle along a stretch of road threatened by mortar fire from IS.

Despite having secured the area a few months previously, IS still harassed the road north with mortar fire and as we were being shown some former defensive positions along a ridge line above the Tigris, a number of explosions and distant cracks of gunfire interrupted our conversation.

With our destination along that road north, we were forced to swap out of our unarmored SUV and into an ex-US Army AMRAP, an armored vehicle designed to withstand IEDs and small arms fire. As we drove north with KSS fighters sticking their weapons out of the portholes and scanning the roadsides for threats, Khazalli explained the fallout from the threats to the highway. "These continued attacks have harmed Samarra and other towns further north, it makes it very difficult to transport fuel, food and medical supplies," he said.

Once the convoy had reached the edges of Samarra, we jumped out and looked over the Samarra dam, which Khazaali claimed IS tried to destroy and instead dumped oil into the water when they failed.

Many Iraq analysts are concerned about the assistance Iran has given these Shia volunteer forces, and whether there are conditions to this assistance once the IS threat has been dealt with. Khazaali however, was quick to dismiss such concerns. "Iran is always welcome to help us, what you have to understand that IS does not only threaten Iraq, they threaten all our neighbours including Iran, there are no conditions to their support."

The convoy moved on before stopping at KSS headquarters, a series of houses next to the dam that was once home to the engineers before the KSS "commandeered" them. Inside one of the homes we met another KSS fighter, Sheikh Ehmad, a tribal sheikh from the southern city of Nusariyeh who happened to be wearing an Operation Iraqi Freedom veteran's hoodie and spoke English, as well as Spanish, Chinese and bizarrely even some Cherokee.

Members of the volunteer force suppress IS positions two to three miles away.

On one of the walls of the living room was an enormous map of the area around the city, displaying KSS and IS positions. "Our forces hold positions on the eastern side of the Tigris in a semi-circle around Samarra, IS attack our positions every day with mortars and machine gun fire," explained Ehmad.

"We have three lines of defence, on the first we have our soldiers armed with small arms, AK-47s, PKM machine guns, RPGs etc, on the second we keep our heavier caliber machine guns like the dushkas and on the third, we have our mortars in position to hit the IS front lines."

It is not just the KSS operating in the area. They also often co-operate with other Shia volunteer forces such as the Badr Brigades and Kata'ib Hezbollah - not to be confused with Hezbollah in Lebanon, " This morning we assisted Hezbollah in an operation to defend the highway heading to Tikrit, we managed to kill 10 IS fighters. They're always trying to cut the road but we stopped them."

Samarra is home to one of the most important shrines in Shia Islam, that of Hasan al Askari, the 11th Shia imam, making it a prime target for IS. The population is majority Sunni, who for centuries have guarded the shrine. When I asked if the KSS's presence in the area was an issue for the local Sunnis, Ehmad denied there were any problems. "We respect them because they have protected our shrine here for centuries and now we protect them, sometimes they even give us information to help fight IS," he said.

Once Ehmad had finished showing us the lay of the land, he asked us to join him on a trip to check on some frontline positions around the village of Sochnas just a few miles north of Samarra. Driving out of the city, the scars of the recent fighting were clear to see, with many burned out vehicles and flattened homes along the roadside.

KSS fighters launch mortars at IS positions less than two miles away.

Our first stop was a mortar position facing north. As we arrived a group of fighters were preparing to fire some mortar rounds at IS positions less than two miles away. Ehmad quickly joined in to help with measuring the distance and once he was satisfied, a round was dropped into the tube. But instead of the ear-splitting bang that we expected, we were greeted with silence, the fighters' faces switching from jubilation to confusion and worry in a split-second. Ehmad was quick to explain: " You see, this is a problem we have, the weapons we receive from the government are old and don't work. We need heavier weapons, anti-tank weapons, if we're to push IS back from its current positions."

After giving up with that mortar tube, they switched to another, giving the round a kiss for good luck and successfully launching it towards its target, firing another shortly after.

Happy with the early evening bombardment, Ehmad took us to some defensive positions a few hundred yards east from the mortar emplacements that acted as a protective buffer for the other position. Along the ridge of earth the fighters had built were a few mannequins dressed in camoflauge gear, "for drawing sniper fire," Ehmad said with a smile. He started pointing out the IS-held villages in the distance and then showed us how he had marked them out on a GPS app on his tablet. "We sent these to the Iraqi air force weeks ago, but still they don't help us," he explained angrily, before ordering the fighters to lay down some suppressive fire on the closest IS village.

A KSS fighter fires at nearby IS positions.

With darkness falling, we were told it was too dangerous to head back to Baghdad on the highway, so we stayed the night at KSS HQ and set off back to capital with Khazaali and some of the other commanders the following afternoon.

Samarra is a small but strategically important city and if IS were to take it and threaten the Askari Shrine, it could provoke another sectarian conflict that couple plunge Iraq into even greater depths.

It was clear from spending a few days with the KSS that without them, IS would have a much greater chance at taking Samarra; while the presence of the Iraqi army is limited, KSS fighters and vehicles are ubiquitous. However, with their supply lines south being threatened and their lack of air cover and heavy weapons, it's unclear whether they'll be able to force IS out of the area completely in the near future.

After hearing of Ehmad's frustrations with the lack of Iraqi aerial support, I asked whether he'd accept any help from the US Air Force. "Of course, I would accept any help from the Americans, we are all on one side to fight IS. Our country needs help. Many Iraqis have been killed."