Biljana Plavsic, a Bosnian Serb biologist and ultranationalist political leader during the Balkan wars of the 1990s who expressed remorse and pleaded guilty to one count of crimes against humanity but later recanted and said she “would do the same again,” has died in Belgrade, the capital of Serbia. She was 96.

The death of Ms. Plavsic was announced on Thursday by Serbian state media. No other details were provided.

Ms. Plavsic, a former professor at Sarajevo University who studied in Zagreb, New York, London and Prague and wrote more than 100 scientific papers, became notorious in the 1990s for applying biology to politics when the multiethnic state of Yugoslavia disintegrated. She declared Bosnia’s Muslim population “deformed genetic material” and railed against mixed marriages, common before the war, as leading to “the degeneration of Serb nationality.”

Often coldly indifferent to the suffering of non-Serbs, she was known as the Iron Lady of the Balkans, a nickname inspired by Soviet descriptions of Britain’s Margaret Thatcher. But the name Ms. Plavsic relished most was a tribute that Serb fighters daubed on the side of their tanks: Empress.

“She was educated, a professor but a real hard-line nationalist,” recalled Laura Silber, the co-author of “The Death of Yugoslavia,” who interviewed Ms. Plavsic several times during the 1992-95 war in Bosnia. “She was prim and proper — school marmish in demeanor — so it was especially shocking to hear the ugly nationalist rhetoric coming out of her mouth.”

Olivera Simic, an Australian scholar from Bosnia who interviewed Ms. Plavsic for hundreds of hours in her later years, remembered her as highly intelligent but often blind to the suffering of non-Serbs. “She was not a monster and would express from time to time that she felt sorry for everyone — and then quickly start talking about Serb victims and cry over them,” Ms. Simic said in a telephone interview.

For nearly four years, Ms. Plavsic served as vice president of Republika Srpska, a Serb enclave in Bosnia that declared itself a separate state in 1992 and began a program of ethnic cleansing aimed at driving out Muslims, Croats and other non-Serb residents. In that role she worked closely with Radovan Karadzic, the self-declared state’s president, who in 2016 was convicted of genocide, war crimes and crimes against humanity by a United Nations tribunal.

She rejected the term “ethnic cleansing” in her interviews with Ms. Simic, a law professor at Griffith University in Australia, describing the separation of different peoples as a “natural process.” According to Ms. Simic, Ms. Plavsic believed that “everyone should live with their own tribe” and, as a result, “willingly turned a blind eye” to the suffering this caused when ethnic groups were separated at gunpoint “because she was only interested in protecting her own tribe.”

Testifying as a witness at the Hague trial of Serbia’s wartime leader Slobodan Milosevic, Vojislav Seselj, the founder of a far-right Serb political party, who was also charged with war crimes, said that Ms. Plavsic “held very extremist positions during the war, insufferably extremist, even for me, and they bothered even me as a declared Serb nationalist.”

After the war ended, Ms. Plavsic muted her anti-Muslim fervor, broke with Mr. Karadzic, accusing him of corruption, and in 1996 took over as president of Republika Srpska with support from the West, which viewed her as a partner for peace.

She was for a time hailed as a symbol of reconciliation after she expressed remorse during a 2002 sentencing hearing in The Hague. “I have now come to the belief and accept the fact that many thousands of innocent people were the victims of an organized, systematic effort to remove Muslims and Croats from the territory claimed by Serbs,” she said.

Serbs, driven by “an obsession to never become victims,” had “allowed ourselves to become victimizers,” she told the court.

Despite her strident ethnonationalism during the war, Ms. Plavsic later won plaudits in Washington and other Western capitals for supporting a peace settlement brokered by the United States in 1995.

Madeleine Albright, who served as secretary of state during President Bill Clinton’s second term, praised Ms. Plavsic at her sentencing hearing for breaking with Bosnian Serb leaders who opposed the peace deal. “She stood up for that at times when it was very difficult, when there were those who wanted to destroy the Dayton Accords,” Ms. Albright said at the 2002 hearing.