By Steve Scherer

CALTAGIRONE, Italy (Reuters) - Street gangs in Tripoli, known as Asma Boys, attacked him with shards of broken glass, slicing his leg and his face, 17-year-old Augustine Okukpon from Nigeria said at a shelter in Sicily.

Okukpon is just one of thousands of unaccompanied minors who have flooded into Italy, many with horror stories about their time in Libya, where people smugglers, militias and Islamic State militants operate with impunity in the chaos of civil war.

He says that his five months stranded there was a nightmare. He was repeatedly attacked and chased by the street gangs and was relieved to climb onto a rubber boat and set off for Europe even if it meant risking his life at sea.

"In Libya there are all those Asma Boys, street boys. They said they don't need any blacks in Libya," he told Reuters at an old villa turned into a shelter for migrant boys in the medieval hilltop town of Caltagirone.

As of May 29, more than 6,000 unaccompanied minors have reached Italy's shores, about three times more than in the same period last year, according to Save the Children.

"People who get on the boats are fleeing an out-of-control situation in Libya that has just been getting worse and worse over the past two years," said Save the Children spokeswoman, Giovanna Di Benedetto, who documents their stories in Sicily.

"Some of them call it hell, and they say they have seen their loved ones exposed to all kinds of violence and even murder," she said.

The United Nations says migrants passing through Libya are often subjected to exploitation and physical abuse, including being held in appalling conditions in detention centers.

Gambian teenager Ebrima Sanneh said the Asma Boys were responsible for shooting dead his Senegalese uncle, who was in his 20s, and wounding his Libyan employer, Mohammed, in the arm.

Sanneh, like Okukpon, arrived in Italy this year by boat from Libya.

"They rob you and take your money, or if they see you have no money on you, they kill you," he said.

Sanneh's employer survived but his arm was amputated. As the man recovered in the hospital, he arranged for Sanneh to take a boat to Italy.

He tried to give Sanneh the 2,500 Libyan dinar ($1,800) he had earned with his uncle in construction work, but Sanneh said he refused.

"He was ready to give me the money. I told him no because he was badly hurt. I told him I'm taking a risk. Maybe if I die at sea, the money sinks."

(Editing by Louise Ireland)