Rooney was also an English global 2003-16, and is the all-time leading scorer for his country.
Newcastle manager Rafael Benitez says the club must break their transfer record - the £16m paid for Michael Owen in 2005 - if they are to sign a striker to score double figures next season.
DC United now sit bottom of the MLS's Eastern Conference after winning just one of their opening seven games of the season.
Despite losing to the Chicago Fire in the 1998 MLS Cup, they stormed back in 1999 to defeat the Los Angeles Galaxy and win their third MLS Cup. The team ultimately ended up trading Adu in December 2006.More news: Bill Gates Says Short Bitcoin, Calls it Worthless Artificial Gold Investment
The MLS club would have to fork out around $17 million for him.
His representative Paul Stretford was sighted in the United States in recent weeks and that has also sparked a lot of interest.
Despite being past his prime, Rooney has performed at such a level this season in the Premier League that he looks to still have at least another really good season in him.
Wayne Rooney is a professional football player who now plays for Everton. Rooney was played mainly as a midfielder, a decision that has aggravated the lifelong Everton fan.
Rooney turned down big-money offers from Major League Soccer in the USA as well as lucrative deals in China 12 months ago.