"Just before we reach the first Daesh checkpoint, everyone throws out their cigarettes," said Abu Ali, a bus driver in his 40s, using a derogatory Arabic name for IS.
"And we spray perfume in the bus so that they can't smell the tobacco, otherwise we'll be whipped!" he said with concern etched into his expression, as he prepared to travel up to Manbij in northern Syria.
The jihadists, who apply their own extreme interpretation of Islam in areas of Syria and Iraq that they have seized since 2013, do more than just inspect the bus for cigarettes.
"They even smell our hands to make sure we haven't smoked," said Jawad, another bus driver, also using a pseudonym for fear of retribution.
Standing near his bus at the Charles Helou station near the Lebanese capital's harbour, another driver begged journalists not to film either his face or his number plate.
"These people, they're dangerous, and they can recognise the bus even from the most minor detail," said the terrified man.
The buses, which leave from a bus station located in one of Beirut's busiest districts, are a rare link between IS-held turf in Syria and the outside world.
'Nescafe, toys, sugar'
Since IS began conquering swathes of Syria in 2013, the buses have made two journeys a week to areas under the jihadists' grip, often with no more than three passengers on board.
In cities like Raqa, the IS de facto capital in northern Syria, and Al-Bab to the west, residents are banned from smoking or wearing clothes deemed inappropriate under the jihadists' interpretation of sharia law.
"Women passengers always carry with them a full-face veil and they put it on before we reach the first IS checkpoint," located on the highway linking Dmeir near Damascus to the ancient city of Palmyra, Abu Ali said.
Men must roll up their trousers to bare their ankles, also in line with IS rules on dress.
Most of the passengers are Syrians going home to towns and cities that have fallen to IS, taking with them food, clothes, medicine and cash for their families.
The drivers also act as delivery men, taking with them goods that are no longer available in parts of war-torn Syria.
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"We take coffee, Nescafe, toys and clothes for the children," said Mohammed, a driver just back in Beirut from Manbij.
Abu Ali says the drivers also carry with them packets of sugar, which has become very expensive.
But "mortadella (cold cuts) is strictly forbidden, because (IS fighters) can't be sure it's halal even if the packaging says it is".
No Kurds, Christians
On this frightening journey to jihadist-held turf, religious diversity is no longer welcome.
"Before, our company would take members of the Assyrian, Syriac, Kurdish and Christian communities. Today, none of them can come on board," Jawad said.
Marwan Zouro, a 55-year-old Kurdish labourer, said he would have to go to Damascus first, and then take a flight to his home city, Qamishli, in northeastern Syria.
The journey is also made life-threatening by frequent fighting and air raids by the regime, Russia and the US-led coalition on areas along the way.
"Before the war, it took us four hours to make this journey; now, the route is 24 hours long," Abu Ali said.
The buses leave from Beirut and pass through Damascus and Dmeir before reaching Palmyra in eastern Syria, in IS hands since spring last year.
The vehicles then head northwest towards Raqa, Manbij, Maskana and Al-Bab near Turkey.
"When the fighting is fierce, the regime doesn't let us through and we are forced to spend one or two nights on the road before we can continue on our journey," Abu Ali explained.
Yet on one of the world's most dangerous routes, neither drivers nor passengers can ever be sure they'll make it to their destinations alive.
"My colleague was heading back several days ago on the Palmyra road, when fighting broke out," Mohammed said.
"I recognised his bus, its front was completely smashed in. He didn't make it."