By Ian Simpson

WASHINGTON (Reuters) - A key witness in the federal perjury trial of former baseball ace Roger Clemens could testify early next week, a prosecutor said on Thursday, setting the stage for a potential courtroom confrontation.

Brian McNamee, Clemens' former personal trainer, could testify "before Tuesday," prosecutor Steven Durham told U.S. District Court Judge Reggie Walton.

McNamee's claims that Clemens, 49, used anabolic steroids and human growth hormone during his 24-year career are at the center of the government's charges that the pitcher lied to Congress about drug use.

Durham has alleged that McNamee injected Clemens with anabolic steroids in 1998, 2000 and 2001 and with human growth hormone in 2001.

Clemens has denied allegations he used performance-enhancing drugs. His defense attorneys have called McNamee a liar.

Former Internal Revenue Service agent Jeff Novitsky, who had investigated McNamee as part of a probe into performance-enhancing drugs, testified on Thursday about evidence obtained during the investigation.

They included needles, syringes, bloody swabs, ampules, vials and other materials turned over by McNamee to federal agents in January 2008.

Clemens' former New York Yankees and Houston Astros teammate Andy Pettitte said in 2007 that McNamee had injected him with human growth hormone in 2002. In a potential blow to the prosecution's case, Pettitte testified on Wednesday that he may have misheard Clemens say he used human growth hormone.

Clemens is being tried for a second time on federal charges of lying to the House of Representatives' Committee on Oversight and Government Reform in 2008 about whether he used anabolic steroids and human growth hormone. The panel was investigating drug use in Major League Baseball.

His first trial ended in a mistrial last year.

Clemens, a seven-time winner of the Cy Young award, baseball's highest annual honor for a pitcher, is among the biggest baseball names linked to alleged drug use.

(Editing by Mohammad Zargham)