Barriot and Crepin, who are testifying for the defence, were members of the French UN contingent in the Croatian towns of Glina and Topusko in 1994, and in 1995 they returned there as activists of a humanitarian agency.
Accompanied by a crew of the French TV station TF3, the two toured the area of the Croatian town of Knin which was then under the control of rebel Serbs, Serb-controlled areas in neighbouring Bosnia-Herzegovina, and in 1998 and 1999 they visited Kosovo and Serbia.
In 1994, the Croatian Serb rebel leader and ICTY indictee, Milan Martic, granted Barriot and Crepin the citizenship and identity cards of the so-called Republic of Serb Krajina (RSK), according to Crepin's testimony.
Barriot, who criticised the then Croatian authorities and media, said that the RSK still existed for him.
"Great injustice was inflicted on Serbs, their suffering was never reported. I have come here to testify on behalf of the Serb people," Barriot, who had to leave the French army because of his pro-Serb engagement, told the tribunal's trial chamber.
Barriot and Crepin, who have published a book on the Krajina Serbs with the introductory note by Martic, made pro-Serb documentaries and interviews with Martic and Bosnian Serb war-time leaders Radovan Karadzic and Ratko Mladic, who are wanted by the ICTY.
Crepin spoke about the suffering of the Serb people in Croatia, Bosnia-Herzegovina and Kosovo and during the 1999 NATO air strikes against the then Yugoslavia.
She spoke about the exodus of Croatian Serbs after the 1995 operation "Storm" and her work in a Serb hospital in Glina in 1994 and 1995, when she got to know about what she called the "wide-spread aggressive conduct of Croats".
Crepin spoke about events from 1991 which prompted the trial chamber's president, Judge Patrick Robinson, to warn her that she was speaking about something that had taken place long before her arrival in Croatia, and was therefore irrelevant for her testimony.
The tribunal's chief prosecutor, Carla Del Ponte, cross-examined the witness, which was the first time she cross-examined a witness before the ICTY.
Del Ponte brought into question Crepin's credibility because of her extreme pro-Serb standpoints.
"Are Karadzic and Mladic heroes for you? Do you know that they are indicted for war crimes?" Del Ponte asked.
Crepin answered that those men had fought for their people and against aggressors and the international community that wanted to divide it.
Barriot claimed that the US army had directly intervened in the 1995 Croatian liberation operation "Storm" by attacking Serb positions in Knin.
The US army intervened without the approval of the UN, Barriot said, adding that the intervention had made it possible for Croatian forces to enter Knin.
The Americans equipped the Croatian army and American officers conducted Operation "Storm", which was confirmed by high-ranking UNPROFOR officers, the French doctor said.
He described Operation "Storm" as horrendous, adding that 2,000 Serbs were killed in massacres and that another 200,000 were expelled.
Barriot is now a member of the French government's committee of experts assessing possible terrorist threats. He has also closely cooperated with the French intelligence services DST and DGSE. He spoke about the role of those services in the release of French pilots who were captured by Bosnian Serbs in 1993.
In 1998 Americans organised the training of Kosovo Liberation Army (KLA) soldiers in cooperation with al-Qaida, whose leader Osama bin Laden was in northern Albania at the time, the witness said, among other things.
Barriot will continue his testimony tomorrow.