Muhammad no longer recognises his country. The 35-year-old former teacher from Idlib province says Syria has been so overrun by foreign fighters that they are the ones calling the shots.

“There are so many foreigners now – I have met guys from Uzbekistan, Ukraine, Libya. It makes me feel like it is not my country any more. Once, I was walking around my home town when a man drove up to ask me for my papers. He was Tunisian. What’s his business ordering me around in my own country, in my town?”

Muhammad’s resentment is shared by many Syrians who have been forced out of their country while foreigners flood in to wage jihad – and also to fight in the ranks of Bashar al-Assad’s regime or in the myriad other militias.

Faisal, 27, also from Idlib, has been working in a Syrian restaurant in Reyhanli, southern Turkey, for more than two years, while watching foreign jihadis travel unhindered through the border town into Syria. “There were so many of them here, all going to my country. These people have ruined us, they have destroyed Syria.”

He accuses foreign powers of supporting without question anyone fighting against Assad. “So many foreign players have their hands in Syria; they are responsible for this. I pray every day that there will be a time when the same troubles will befall them.”

A UN security council report obtained by the Guardian says at least 15,000 people from more than 80 countries have travelled to Iraq and Syria in recent years to become jihadi fighters. Armed opposition groups initially welcomed foreign fighters to Syria, but their growing influence, religious fervour and violence have alienated ordinary Syrians, many of whom feel the jihadis are part of an attempt to further destabilise the country from outside.

It is no secret that Sunni states in the region have long supported and funded armed opposition groups in Syria and Iraq, though the US vice-president, Joe Biden, speaking at Harvard in October, caused a stir between the US administration and its allies when he accused Turkey, Saudi Arabia and the UAE – previously lauded in the fight against terrorism by Barack Obama – of pushing for “a proxy Sunni-Shia war” in Syria by providing financial, military and logistical support for “the extremist elements”.

The foreigners fighting in Syria had little trouble entering the country through the 550-mile border with Turkey via what Turkish pundits called the “jihadi highway”. Working not unlike regular tour operators, traffickers ran routine – and lucrative – transfers from Turkish airports close to the Syrian border while the authorities and border guards turned a blind eye.

“For the first two years of the conflict in Syria there was virtually no border,” says Ahmet, a smuggler and lifelong resident of a border village in Hatay province. “We pretty much came and went as we pleased. The Turkish government didn’t seem to mind.”

At the height of jihadi traffic in 2012, he sometimes ran three tours a day from the airport and the bus station in Hatay to Syria. An ethnic Arab, Ahmet says most of the men he smuggled into Syria did not speak Arabic, and many appeared to be religiously conservative: “I brought in a lot of very religious guys, and I really liked that. They reminded me of my own shortfalls as a Muslim.”

Facebook Twitter Pinterest Militants en route to the Syria-Turkey border in the Hatay province of Turkey. Photograph: Jonathan Lewis

Under mounting US pressure, Turkey has increased border security, cracking down on smuggling and tightening rules for Syrian refugees trying to enter the country. Ahmet confirms that known smuggling routes have become harder to access. “I stopped smuggling foreign fighters into Syria because it had started to deteriorate. I didn’t know who these people were, whom I was taking into Syria and what they were here for,” he said. “It became shadier, I wasn’t sure any more of the people coming in.”

The unease is global. This year, the UN security council passed a number of resolutions urging member states to step up screening measures and border patrols aimed at stemming the flow of foreign fighters to Iraqi and Syrian battlefields. Muhammad, the teacher from Idlib, scoffs at such international efforts. “Without foreign support, these groups would never have grown this powerful in Syria,” he says. He now lives in Reyhanli with his wife and two children while regularly returning to Syria to visit his parents who refuse to leave home. He says he often comes across foreigners, some of whom are unable to communicate in Arabic.

Despite this, he thinks the global focus on foreign jihadis fighting for Islamic State (Isis) and al-Qaida-affiliated groups such as al-Nusra Front is one-sided: “There are many foreigners in Syria fighting for the other side. There are Iranians, Russians and Lebanese who fight for Assad. What about them? The other foreigners come to help us, because their governments don’t.”

Abu Nour, 35, a primary school teacher and former fighter for the jihadi Ahrar al-Sham from Aleppo, says all foreign combatants were initially welcomed by the armed opposition, but no longer: “They provided decisive support in many battles. We were desperate for anyone to help us, but nobody – not the UN, not Nato, not even other Arab states – stepped up to do so. So the foreigners came. Some of them are good, they want to fight Assad and help us, but many have turned bad. They come for the money, for women. They destroyed the revolution.”

Abu Obaydah, 28, a Syrian fighter for al-Nusra Front in Aleppo, says most foreigners fighting for al-Nusra left for Isis when the two groups fell out last year. “Most foreigners don’t understand the reality on the ground in Syria,” he says. “They hear about it in the mass media and on the internet. So it is easy for groups like Isis to teach them whatever they want and to brainwash them.”

While jihadi groups such as al-Nusra Front and Ahrar al-Sham boast professional social media presences and have made names for themselves, neither can drum up as much attention as Isis. “The international media have done their bit to make Isis this famous,” Abu Obaydah says. “That makes it part of the attraction for foreigners who want to come to this so-called Islamic State (Isis), but who have little knowledge of the real Syria.”

Most foreigners are suspected of joining the ranks of Isis, now the principal jihadi group in Syria and Iraq, whereas the Free Syrian Army (FSA), an umbrella term for a network of armed opposition groups deemed moderate by the US-led military coalition, draws on only Syrian fighters. The People’s Defence Corps (YPG), the Syrian-Kurdish force that has been defending the town of Kobani against an Isis attack since mid-September, is the only non-jihadi group known to have attracted foreigners.

Facebook Twitter Pinterest The Free Syrian Army – a network of armed opposition groups deemed moderate by the US-led coalition, prefers not to use foreign fighters. Photograph: Hamid Khatib/Reuters

Samir, 28, from Lattakia province, is affiliated to an FSA battalion fighting close to the Turkish border. He cites the lack of a strong ideology and reward prospects as reasons why his group remains all-Syrian. “These fighters often come because of what they believe in, because they come for jihad, not for the revolution itself. The FSA has little to offer there, it is just not as attractive.”

Abu Ayman al-Ansari, 30, formerly a surgery assistant from Hama and now a chief medical officer for the Chechen-led jihadi Jaish al-Muhajireen wal Ansar (Army of Emigrants and Supporters, JMA), agrees: “The FSA fights for democracy, but jihadi groups want Islamic law and unity among Muslims. This is a goal many foreigners support.”

Like other jihadis, he rejects nationalism and so is less troubled by foreigners fighting alongside Syrians. “We were immediately impressed by the Chechen commanders,” he recalls. “They and their sons were always fighting on the frontlines. That made us trust and believe them.”

Abu Ayman says that many of his fellow fighters placed similar trust in the Chechens’ religious convictions: “Our faith was skewered by government influence, but their way of religion is right, it’s unspoiled, because they learned it without outside interference, it’s purer. They support Muslims everywhere.”

Many of the JMA fighters, a group formerly headed by Omar al-Shishani, now a senior leader of Isis, come from Caucasian and central Asian countries, he explains, though one JMA battalion unites jihadis from western countries – the US, UK, Germany and others – who fight together “for language reasons”.

Abu Ayman says jihadis do not wish to quit the war and return to their countries. “They come because they have a very strong religious conviction. Many foreigners I know volunteer for suicide missions against Assad forces.”

But the international focus on foreign jihadis has made their recruiters wary. Abu Obaydah underlines that any foreigner aspiring to fight with his group needs a recommendation from another foreign al-Nusra Front member, adding that some jihadi outfits have set up monitoring groups that perform background checks on fighters coming from abroad.

“We have caught several spies posing as fighters,” he said. The JMA sends foreigners to fight on the frontlines, without exception. “Those that who refuse are immediately suspect.”

Only those who have gained the trust of the leadership are sent abroad for recruitment and to raise financial and logistical support.

“But those travelling out of Syria don’t wear those beards,” the medical officer says, smiling. “They wear earrings and often look quite fashionable.”

“Foreigners who enter Syria are often watched and followed by anti-terrorism groups and secret intelligence,” says Abu Ayman, who liaises between the opposition factions in Syria to organise ambulances and medical care. “This is one reason why we try never to send wounded foreign fighters to Turkey – most of them are being treated in Syria.”

Most foreigners are suspected of joining Isis militants, above, who now form the principal jihadi group in Syria. Photograph: Medyan Dairieh/Zuma Press/Corbis

Scores of Syrian fighters have been treated in Turkish hospitals since the start of the war, and private clinics and rehabilitation centres have been set up in many Turkish border cities. But Abu Ayman increasingly prefers to tell border guards that the wounded he brings across the border are with the FSA. “Everyone has become more nervous,” he says. “Even the Turks.”

FSA fighter Samir is angry that foreign fighters in Syria put themselves in charge and try to impose their religious views on the locals. In his experience, Tunisians, Saudis and Iraqis are among the most ruthless.

“These people don’t know Syria, and don’t understand it,” he says. “Why should our women suddenly be dressed all in black?”

A heavy smoker, he is not happy with the new regulations enforced by fighters of al-Nusra Front in his town either. “We will welcome anyone who wants to fight against Daesh [Isis] with us. But our religion does not allow killing indiscriminately and it doesn’t ban smoking. We would like people to come and help us in our fight, but they need to come to the right place.”

Sami Laani, an opposition journalist from the eastern province of Deir ez-Zor, describes an all-female police force formed by Isis in his city. “They watch the women, how they dress and behave. None of them are Syrian, but they think they know better than us.”

In his eyes, the foreigners fighting in Syria are an occupation force, but he blames fellow Syrians for allowing them in: “All these foreigners would never be able to come and do what they do without the help of Syrians, who know the country,” he says bitterly. “I hate those Syrians even more than I hate those foreign fighters. Why do they help them to destroy Syria?”

• Some names have been changed