The man, whose name has not been released, is believed to be holding several other children, The Times of Trenton reported. By late Saturday afternoon, he had been holed up in his home in South Trenton for 24 hours with his girlfriend's body.
The streets around the house were blocked off and police officers, joined by Mercer County sheriff's deputies and state police officers, kept watch. FBI agents were also on the scene.
Trenton Police Lt. Steve Varn said police were at a "standstill" as they attempted to negotiate with the man. First Assistant Mercer County Prosecutor Angelo Onofri said those involved still hoped for a surrender.
"We are still working toward a peaceful resolution," Onofri said.
The suspect had been holding police at bay since Friday afternoon when officers checking the welfare of the residents discovered the body of a woman and then retreated from the house when a man with a gun appeared.
The unidentified suspect apparently had killed his girlfriend and one of her children inside the home, law enforcement sources told the Times.
Neighbors said the woman had five kids, all in their teens except a 4-year-old son. The eldest, a 19-year-old autistic boy, was reported safe with relatives.
The Star-Ledger of Newark said a heavy overnight rain kept activity outside the house to a minimum, and officers reported no sounds coming from the house.