MIAMI • The World Anti-Doping Agency (Wada) on Monday described allegations made in an Al-Jazeera report that Denver Broncos quarter-back Peyton Manning took human growth hormone (HGH) as "very concerning" and has urged "increased collaboration" with sports leagues in the United States.

Though Manning has vehemently denied the allegations and the source of the Al-Jazeera report has since recanted the claims, Wada says that it expects a careful investigation by the relevant authorities.

The Dec 27 investigative documentary, The Dark Side: The Secret World Of Sports Doping, linked well-known players from the National Football League (NFL) and Major League Baseball (MLB) with performance-enhancing drugs.

Green Bay Packers linebackers Clay Matthews, Julius Peppers and Mike Neal, whom the report said have used banned hormone supplement Delta-2, also denied any wrongdoing.

David Howman, director-general of Wada, said he was nevertheless taking the report seriously.

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

While the documentary's makers and its critics have clashed over the validity of the allegations, the report has nevertheless pushed concerns of illegal performance enhancements in the US' most popular sports leagues into the spotlight once again.

Al-Jazeera's report also highlighted the long-running differences between Wada and the NFL in its anti-doping strategy.

"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 programme," Howman said.

NFL spokesman Brian McCarthy said: "Regarding Mr Howman's statement, we have valued our long-standing association with Wada and look forward to work closely to improve the effectiveness of all anti-doping programmes."

The MLB said it would investigate allegations made in the documentary that several of its players took banned hormone supplement Delta-2. Among the baseball players named were Ryan Howard of the Philadelphia Phillies and Ryan Zimmerman of the Washington Nationals.

REUTERS