Al-Laffa - It was Efrem Desta's yearning for freedom that made him flee his home country of Eritrea and enter Sudan illegally, hoping that he could later make it to Europe.
But he and a group of fellow migrants were abducted by Sudanese Bedouin Rashaida tribesmen after they crossed into east Sudan near Al-Laffa village.
"We fled Eritrea because we wanted freedom, but when we got here we were captured by Rashaida," said Desta, 20, speaking in his native Tigrinya language.
"After five days in captivity, we were rescued."
Sudanese security forces, who have stepped up their patrols along the 600km frontier with Eritrea in a bid to curb migrant smuggling, freed the group.
They were found handcuffed and in chains, security officers said, and have now joined nearly 30 000 other refugees in Wadi Sherifay camp, a vast conglomerate of thatched huts and dusty tracks near the border.
Most of the rescued Eritreans say they fled their country to escape military conscription, but some do admit leaving to seek better jobs abroad.
Sudanese police and agents of the powerful National Intelligence and Security Service (NISS) say dozens of Eritreans try to enter Sudan illegally every day.
"There are many ways they enter, including walking along the river Gash," one security officer told an AFP correspondent who toured border areas of Kasala state at the beginning of May.
The migrants cross into Sudan on foot after walking for days or in some cases even weeks.
Key transit point
"They usually travel at night and hide out during the day in farms, plantations and forests," the officer said, pointing to a patch of trees lining the dry riverbed.
Although Syrians fleeing their brutal civil war fuel the current migration crisis, experts say there are also many Eritreans trying to reach Europe.
"An estimated 100 000 migrants travelled across Sudan in 2016, the bulk of them being Eritreans," said Asfand Waqar, analyst at the International Organization of Migration (IOM).
Sudan, in the Horn of Africa, is a key transit point on the migrant route to Europe.
From Kasala the Eritreans travel across Sudan to Libya or Egypt.
Smugglers then cram them aboard rickety boats for perilous Mediterranean voyages aimed at reaching landfall in Europe.
In summer, the long windswept cross-border Gash riverbed comes alive at night with the march of migrants.
"We still don't do night patrols, so it's easy for them to move during the hours of darkness," the security officer said.
Behind him under the scorching midday sun, a group of machinegun-toting border guards crossed the riverbed in pick-up trucks to begin a patrol.
Officers say that their boosted presence along the border had also helped them catch several people smugglers.
"The smugglers, who are mostly Eritrean, have excellent networks and high-tech communications gear," another security officer said.
"They know more about us than we know about them."
Migrant smuggling has become a multi-billion-dollar business, experts say.
"It's the financial capability of a migrant that determines how much he would be charged. It's an exploitative system," said Waqar, with the cost ranging from hundreds to thousands of dollars.
An Eritrean woman planning to travel to Europe from Khartoum said she was told to raise $2 500.
Kasala police chief General Yahya Sulayman said Sudan alone cannot stop the smuggling of people along the "long and complicated" border.
"We need international help, hi-tech communication equipment, vehicles, cameras and even drones to monitor the border," he told AFP.
Washington-based think tank Enough Project says the European Union paid Khartoum millions of euros to buy equipment that would help stem the migrant flow.
Some funding also went to the Rapid Support Force (RSF), a paramilitary group fighting rebels in war-torn Darfur, and whose members are also used for border patrols, the think tank said.
General Sulayman denied that any RSF members were deployed along the Sudan-Eritrea border.
"The border patrols are carried out by police, NISS and Sudanese armed forces," he said.
"All these troops are jointly fighting organised cross-border crime."
Eritreans in camps such as Wadi Sherifay say they live in a constant state of fear.
"The Eritrean military has its agents everywhere. They can catch us and take us back," said one who still dreams of reaching Europe.
"It's not safe for us to be here for long."