The Justice Department is stepping its technical assistance to Atlanta law enforcement agencies in an attempt to solve the murders of at least 15 black children, Vice President Bush announced yesterday.

Bush, designated by President Reagan to coordinate the federal effort, did not commit the FBI or its resources, nor did he respond to a call Friday from civil rights leader Joseph Lowery for Reagan to declare the city eligible for federal disaster relief funds because of the number of child deaths.

Bush said the Law Enforcement Assistance Administration has developed a plan to provide technical assistance in the murder investigation, and the office of juvenile justice and delinquency prevention will provide what advice it can.

In Atlanta, an examination of the body of the latest victim found, 14-year-old Lubie Geter, has provided a clue that may "tie together more" the mysterious deaths over the last year and a half, a medical examiner says.

Whoever strangled the boy left no marks on his neck, said Dr. Robert Stivers, a Fulton County medical examiner. "It looks like a choke hold," Stivers said, "someone coming up from behind and catching the child in the crook of their elbow.

"Some of these are beginning to tie together more. It's beginning to make a believer out of me."