The World Anti-Doping Agency (WADA) has called on the NFL and MLB to investigate alleged use of performance-enhancing drugs (PEDs) in its leagues.

An Al Jazeera documentary, which aired on December 27, linked players from MLB and the NFL to PEDs.

WADA director general David Howman said the allegations were worrying and called for an investigation, despite neither being a signatory of the body's code.

"Al Jazeera's allegations are very concerning; particularly, as it relates to the NFL's and MLB's testing programs," he said in a statement.

"While the NFL and the MLB are not signatories to the World Anti-Doping Code; in recent years, WADA has been working with them, and other professional leagues in the United States, to try to bring them closer to WADA's program.

"In particular with the NFL, we have been offering guidance to enhance, and increase the transparency of, their testing program.

"We would of course welcome increased collaboration with the leagues and their Players’ Associations to discuss appropriate enhancements that could be made in support of clean athletes."

Howman added: "As it relates to the particular allegations by Al Jazeera, WADA expects that they will be carefully investigated by the relevant authorities and that, if warranted, necessary and appropriate steps would be taken.

"While the matter is outside of WADA’s mandate; as always, the Agency stands ready and willing to work with authorities as appropriate."