When Um Ahmed heard an explosion on June 20, she worried for her two sons' safety. A neighbor of hers in Yemen's capital city of Sanaa told her a car bomb had detonated next to the historic al-Mahdi mosque on the outskirts of Sanaa's Old City.

"It was the mosque my sons prayed at every week," she told VICE News. "I ran there immediately."

Ten minutes later she was at the mosque, a scene of bloody destruction, but the dead and the injured had already been taken to nearby hospitals. She spotted her son Ahmed's friend, sitting on a motorcycle. He was weeping.

"Why are you crying?" she asked him.

"Because Ahmed is dead."

Fortunately, he turned out to be wrong: Ahmed, 18, and his brother Mohammed, 15, had been injured, but they survived the bombing. Mohammed suffered burns and shrapnel wounds, and now spends most days lying in the cushioned mafraj, a sitting room at the front of his mother's home; neither boy is yet able to leave the house due to their injuries.

The bombing forced Um Ahmed to make what, for a conservative Muslim, was a difficult decision: She stopped attending prayers at the mosque.

She's not alone. Other Yemenis have stopped venturing out as much as they did before the war. Months of conflict have sewn distrust among neighbors and created an atmosphere of insecurity throughout the country, leaving room for radical jihadist groups like al Qaeda and the Islamic State (IS) to fill the void.

Yemeni men survey the aftermath of a car bomb near Qubbat al Mahdi Mosque in Sana'a, Yemen on June 20, 2015. The bombing was later claimed by Yemen's newly formed branch of ISIS.

Mohammad and Ahmad al Asabah sit together in their home in Sana'a, Yemen on July 4, 2015. Both were injured and hospitalized with severe burns and shrapnel wounds from the ISIS-claimed car bomb near Qubbat al Mahdi Mosque.

The IS branch in Yemen claimed responsibility for the attack on the al-Mahdi mosque, which is known as a place of worship for followers of Zaydism, a form of Shiite Islam largely unique to north Yemen. Zaydism is key to the ideology of the Houthis, a north Yemeni militia that last autumn seized Sanaa and now controls much of the western half of the country. IS justified the bombing as revenge for Houthi incursions into Sunni Muslim areas of Yemen.

The Houthis started out as a revivalist movement for Zaydism but morphed into a powerful militia during six wars with the central government between 2004 and 2010, sparked by the death of their founder-leader Hussein al-Houthi at the hands of Yemeni security forces. At the time, then-president Ali Abdullah Saleh saw them as a threat; now he is working alongside them to quash the Sunni Islamists who helped oust him in 2011 and win back control of Yemen through force. The Houthis have proven surprisingly willing to put the past behind them as they push to become the country's dominant power alongside Saleh and his allies.

The attack on the al-Mahdi mosque was the sixth such bombing ofa mosque known for attracting Zaydi worshipers that IS has claimed responsibility for, including two suicide attacks in March that killed 130 people. IS says Yemen's Zaydis,who are largely concentrated in the northern highlands, including Sana'a, are apostates and heretics, and calls on Yemen's Sunnis to wage holy war against them.

The IS attacks appear designed to create discord between Yemen's Sunnis and Shias, according to Katherine Zimmerman, an al Qaeda analyst at the conservative American Enterprise Institute and a Yemen specialist. She says the IS strategy is in line with the sectarian divide-and-conquer technique the group has employed to spectacular effect in Iraq and Syria.

"The Islamic State is growing in the areas where the Houthis are," she said. "If it can push Yemen towards greater sectarianism, then IS will thrive."

But Yemenis argue that IS has yet to achieve the same success it has had in Iraq and Syria, because of Yemen's more ingrained culture of religious coexistence.

That said, IS does appear to be achieving its goal of fermenting distrust, if not sectarian hatred.

"To start with, we just feared death from above," said one resident of Sana'a, referring to the ongoing campaign of Saudi airstrikes against the Houthis. "But now we fear it from all around." Neighbors who have lived alongside one another for years now eye each other with mutual suspicion, he says, wondering if they are quietly involved with IS or al Qaeda.

But not all Sana's residents share this view.

"We fear airstrikes, explosions, terrorists, but we do not fear each other," Abdullah Ghara,who prays regularly at the al-Mahdi mosque despite the mounting threat of attack, told VICENews. "Zaydi, Shafei, Sunni, we are all brothers… These groups [IS and al Qaeda] can only attract small numbers of people, people who are weak of faith and do not know their history. But they cannot recruit Yemenis, who know that we've all lived together peacefully for thousands of years."

Fuad al-Attar, who also attends the mosque in spite of the risk, told VICE News that while Yemen was divided down political and geographical lines, religion played no part in the country's ongoing civil war, and blamed extremists from Saudi Arabia for the mosque bombings.

"No Yemeni would ever blow himself up," he said. "The people who are doing this are sent from Saudi Arabia."

A Yemeni woman raises here hands in a peace symbol during a rally for Southern Independence in Mukalla, Hadramout, Yemen on July 6, 2013.

For the most part, Yemenis are resistant to a narrative of sectarian divisions.

Yet while men like Ghara and al-Attar refute the idea of sectarian divides, they cannot resist vilifying Islah — Yemen's main Sunni Islamist party — and Saudi Arabia, which promotes the ultraconservative Wahabist Sunni doctrine. There is no sectarian divide in Yemen, the message is, because the people promoting sectarian violence are not real Yemenis.

Last September, with the help of Saleh loyalists,the Houthis descended from their stronghold in the north of the country and seized the capital, Sana'a, with the tacit support of Ali Abdullah Saleh, Yemen's wily ex-president, who was ousted during the Arab uprisings of 2011. They signed a peace deal with Abd Rabbu Mansour Hadi, the man who replaced Saleh as president in 2012, but in January placed him under house arrest, accusing him of trying to split the country into a new federal state structure that they were convinced was designed to weaken them. In February, Hadi fled to the southern port city of Aden, vowing to return to Sana'a and oust the Houthis. In return, the Houthis, backed by Saleh loyalists, besieged Aden, forcing Hadi into exile and sparking the current war.

This mounting distrust has arisen in a power vacuum.The war has seen the state and military collapse as the Houthis and Saleh loyalists together fight rival militias, who want to prevent the Houthis from entering the areas under their control. Many, but not all of them are Sunnis, nominally allied with President Hadi, who is being supported by the Saudis, leading a coalition that is attempting to bomb the Houthis into submission while sealing the country's land and sea borders.

These alliances do not fit neatly into a Sunni-Shia sectarian divide IS hopes to deepen. The war has ultimately been a struggle for power as much between highlanders from the northwest and rival groups in the west and south. But the fear is that the expansion of the Houthis into territories that are majority Sunni will push their rivals into the arms of radical Sunni jihadist groups like IS and Al Qaeda in the Arabian Peninsula (AQAP).

In March, soldiers and government workers fled the southeastern port city of Mukalla, fearing that the Houthis, who were advancing on Aden, another southern port, would soon be on their doorstep. AQAP quickly seized the city, striking up a cooperative agreement with local tribes, rebranding its fighters as "Sons of Hadramawt" — the name of the province Mukalla is part of, one of the biggest oil-producing regions in Yemen —and handing over the day-to-day running of the port to a council made up of local leaders. People in the largely Sunni area saw little choice but to accept AQAP's authority, says a resident of Mukalla, pointing to the vicious tactics employed by the Houthis as they try to bludgeon their rivals into acquiesecence, which have included indiscriminate shelling in cities like Aden and Taiz.

Supporters of the Houthis sing nationalistic songs at a gathering to celebrate their ousting of the former government in Sana'a, Yemen on February 7, 2015.

Yet the arrival of IS in Yemen — which made formal its Yemen debut with a mosque attack in March that killed 130 people and injured more than 300 — could cost AQAP dearly, forcing it into a fight with rival jihadists or pushing it into employing ever more brutal tactics. In Syria the main al Qaeda group, Jabhat al-Nusra, has spent almost as much time fighting IS_, _which split from al Qaeda in early 2014, as it has the regime of Bashar al-Assad.

AQAP has adopted something like a hearts and minds approach on the ground in Yemen, and is generally careful to avoid civilian casualties, meaning that its approach is at odds with IS's _modus operandi _of rule through fear. AQAP has condemned the mosque bombings and in December 2014, a senior AQAP leader released a video message criticizing the tactic of beheading prisoners for propaganda purposes, in a rebuke to both IS and to one of its own men who had publicly executed 14 soldiers earlier in the year.

But fears of mass defections to IS, as happened in Syria, and recent changes in AQAP's leadership could also lead to a shift in tactics, in turn adding fuel to the fire of mistrust and resentment between different communities. AQAP has been set back in recent months by the death of both its leader and al Qaeda second-in-command, Nasir al-Wuhayshi, and operations chief, Nasr ibn Ali al-Ansi, in US drone strikes. Wuhayshi was replaced by Qasem al-Raymi, the group's top military commander.

"I think the leadership is in reset mode," says the American Enterprise Institute's Zimmerman.

Hisham al-Omeisy, a Sana'a-based political and information analyst, says al-Wuhayshi was a charismatic leader who opposed the brutal ways of IS and ensured cooler heads prevailed when the younger generation of AQAP felt that without violence, they would be outdone by IS.

"There's a high probability of AQAP adopting a more aggressive operational stance," he said.

IS and AQAP do not yet operate in the same areas of Yemen, Zimmerman adds, meaning that they are yet to come into direct conflict with each other. The small Yemeni wing of IS operates in the northwest and west of the country, with limited support, while AQAP's main base is in the south and the east. But in the longer term, she says, IS will likely try to compete with AQAP, creating potential for a replay of the jihadist infighting that has created chaos in parts of Syria.

Western security officials worry that the rise of both AQAP and IS in Yemen could fuse violent radical Islamists into Yemen's tribal system in a way that has hitherto not been possible.

A Western diplomat told VICE News that Washington and London are concerned that Saudi Arabia, which has driven the international response to the Houthis' rise, seems worryingly blasé about the growth of AQAP and IS.The Saudis see the Houthis as a proxy for Iran, their main regional rival, and as such have made their defeat a priority in Yemen.

"The message we are hearing is that they have other priorities — the Houthis — and we are welcome to do what we want about AQAP and others," said the official, who requested anonymity since he was not authorized to speak publicly. He pointed to the problems Western governments have had in Libya and Syria, where the single-minded focus on bringing down hostile regimes has seen the Arab Gulf states directly and indirectly arming extremist groups.