Worldwide deaths of children under 5 had fallen below 10 million in 2006, it had said, providing some self-congratulatory moments for health administrators.
"Both of these examples show how U.N. agencies are willing to play fast and loose with scientific findings in order to further their own institutional interests," the Lancet said in an editorial.
"The danger is that by appearing to manipulate science, breach trust, resist competition, and reject accountability, WHO and UNICEF are acting contrary to responsible scientific norms that one would have expected U.N. technical agencies to uphold. Worse, they risk inadvertently corroding their own long-term credibility," according to the Lancet editorial.
It further noted that the UNICEF generally published its child mortality data only in December but this year rushed this year's release to make it public before Lancet came out with a more critical assessment.
In the assessment published in Lancet, Christopher Murray, a professor of global health at the University of Washington and a former WHO official, estimated that between 9.5 and 10 million children under 5 died in 2005.
His team concluded there had been too little progress in reducing child deaths, writing, "Globally, we are not doing a better job of reducing child mortality now than we were three decades ago."
But a UNICEF spokeswoman said the agency had done nothing wrong in releasing the child mortality figures as it did.
"UNICEF first announced that the under-5 mortality figures were likely to fall below 10 million at two major conferences in June," the agency said in a statement.
"As soon as we had confidence in a more precise figure (9.7 million), we also made this available. UNICEF hopes that the progress revealed by the new figure will act as a spur for greater urgency to achieve the child survival goals," the statement added.
UNICEF said last week global efforts to promote childhood immunization, breast-feeding and anti-malaria measures had helped cut the death rate of children under age 5 by nearly a quarter since 1990 and more than 60 percent since 1960.
The Lancet also faulted the way the World Health Organization used research data on a key method of preventing malaria -- using bed nets treated with insecticides to ward off the mosquitoes that spread the disease.
The Lancet editorial said WHO ignored certain limitations in the study in making a public statement about the use of the bed nets.