U.S. drug reviewers have recommended additional study of delayed development reported in children of mothers who took Abbott Laboratories' anti-seizure drug Depakote, a memo released on Friday said.

Six cases including two siblings reported to have autism "raise concerns," Food and Drug Administration staff wrote in a May 6 memo.

Limitations in the reports "make it impossible to definitively conclude" the drug played a role, and other factors may have contributed, especially in the two sibling cases of autism, the FDA reviewers said.

"Further study is needed to further elucidate causality and degree of risk of fetal exposure to (Depakote) and subsequent developmental delay in children," they added.

An Abbott spokeswoman could not immediately be reached for comment.