A Christian ministry in the Middle East has expressed alarm over the growing violence and intimidation inside refugee camps reportedly carried out by Islamist gangs and Syrian militants who have been hiding among the refugees displaced by war in the Middle East.

Christian Aid Mission said Islamist gangs have brought practices into the UN refugee camps such as "coercion to join terrorist groups such as the Islamic State (ISIS), conflict between militias on both sides of the civil war and the criminal buying and selling of females as sex slaves.''

"The Muslim gangs come as refugees, but they have their agendas," the ministry director, whose name was withheld for security reasons, told the ABC News. "They're like a mafia. People are even killed inside the camps, and the refugees are afraid to say if they saw somebody get killed. If you ask them, they'll say, 'I don't know, I was asleep.'"

"The last time I went inside a camp, I had a policeman with me," the ministry director said. "The camps are dangerous because they have ISIS, Iraqi militias and Syrian militias. It's another place for gangs. They're killing inside the camps, and they're buying and selling ladies and even girls."

Inside the camps, he said ISIS militants urge refugees to swear allegiance to the caliphate or be killed — a tactic members of the jihadist group openly practices in areas it has captured.

These gangs also reportedly execute converts who are not sincere with the religion and behead their friends or people who hid them.

Syrian and Iraqi women refugees are likewise subjected to several abuses by sexual predators inside camps.

Since seizing large swaths of Iraq and Syria in an attempt to establish an Islamic caliphate, ISIS has displaced hundreds of thousands of citizens who were forced to reside in refugee camps. To date, about 20 percent of the Syrian refugees displaced by the group in Jordan are living in such camps, which offer very little protection, said the Gospelheard ministry.

The ministry, which was started in 1990 to bring both physical and spiritual aid to several countries in the Middle East, has 32 full-time workers and 400 volunteers who work to share the good news of Christ's salvation despite danger to themselves.

"We have had to follow some clever methods that outweigh the intelligence of any of us," the ministry director said. "We certainly know that God is behind these methods. We have witnessed the fruit and many great experiences throughout these years that made us realise that God is the one leading us."