An undercover cop fatally shot a drug-dealing, teen ex-con in The Bronx yesterday after the youth tried to rob him during a sting operation, police said.

Nelson Reeves, 17, allegedly demanded more cash for crack and fired one shot from his .25-caliber gun when he met the undercover at 11:20 a.m. in the vestibule of 1409 Prospect Ave. in Morrisania.

The cop blasted back four times, police added.

Reeves, who was released from prison in May after serving eight months on a gun rap, was pronounced dead at Lincoln Hospital, cops said.