Former NATO commander and potential presidential candidate Wesley Clark started his testimony at Slobodan Milosevic's trial at The Hague war crimes tribunal on Monday – but not publicly.
The US Democratic nominee's evidence is being heard behind closed doors, according to tribunal spokesman Jim Landale.
Clark directed NATO's 11-week bombing campaign against Serbia in 1999 during Belgrade's crackdown on Kosovo Albanian separatists.
He is one of the most high-profile figures to be called to testify at the trial of the former Yugoslav president.
The retired four-star general was also part of the US team which helped to negotiate the 1995 peace agreement ending the Bosnian war.
His testimony will continue on Tuesday, and his evidence is to be made public later this week.
Washington has effectively delayed publication of his evidence until Friday.
Milosevic has been on trial since February 2002, charged with ethnic cleansing in Croatia, Bosnia and Kosovo in the 1990s.
He is defending himself against charges of genocide, crimes against humanity and war crimes.
Clark spoke to Milosevic for more than 100 hours over a period of almost four years in the 1990s during former US President Bill Clinton's time in office.
A Vietnam war veteran and staunchly anti-Iraq war, Clark is attempting to win the Democratic nomination to take on Republican President George Bush in the 2004 White House race.