Two men and three teenage boys have been charged with gang-raping a seven-year-old girl sold for sex by her 15-year-old stepsister during a party at a crime-ridden apartment building in New Jersey.

Details of the arrests were announced outside police headquarters in Trenton last night.

Police director Irving Bradley said detectives had been working around the clock since the crime was reported on March 28.

"We did get a lot of co-operation from the community, which helped break this case," Mr Bradley said.

Those arrested included Gregory Leary, 20, in custody since Friday, when he was charged with having sex with the 15 year old.

The other four were Timear Lewis, 19, and boys aged 13, 14 and 17. Each was charged with aggravated sexual assault and child endangerment.

Prosecutors will probably seek to try all of them as adults, Mercer County prosecutor Joseph Bocchini said.

The case shocked residents of the Rowan Towers apartment building, where three of those arrested lived, police captain Joseph Juniak said.

The building's management has stepped up its security presence there and promised additional safety measures, but residents expressed scepticism that the flats would become safer.

Police said the 15-year-old girl went to a party with some men at Rowan Towers on March 28 and the younger girl tagged along because she was worried about her stepsister's safety.

They say the 15-year-old sold sex to men and boys there, then took money to let them touch the younger girl. They say the touching turned to forcible sex as at least seven men raped the seven year old.

Additional arrests were expected, police said.

Before the arrest announcement, the 15-year-old girl had been charged with promoting prostitution, aggravated sexual assault and other crimes. She remained in juvenile detention today.

Trenton mayor Doug Palmer, who met the seven-year-old victim and her family in his office, described their ordeal as a "torment".

"They're in a safe place right now," he said. "I would ask that the community embrace this family - they are going through a lot."

Mr Palmer, who gave the girl a stuffed rabbit for Easter, said the family was having counselling and indicated it may be moved permanently from the city.

He praised police for their hard work but said the investigation into "this heinous crime" was not over.

"We're not finished because everyone who is responsible is still not arrested," he said. "The police will not rest until we get every individual who was involved in this."

Mr Palmer said he also met the girl's grandmother, two younger children and other relatives.

"They were understandably upset," he said. "They felt like they were victims. They said, 'People are blaming us'. We need to stop pointing fingers at the family members. These rapists are the problem, not the family."

Police earlier urged residents to come forward, but neighbours said they were scared of retaliation from gangs that prowl the streets if they spoke up.

Rowan Towers is on a stretch of West State Street near central Trenton and is surrounded by blocks of abandoned, boarded-up homes.

City councillor Annette Lartique, who represents the area where the crime occurred, said the high-rise had been plagued by crime.