"We have advised the defence to immediately take action to file an appeal against the verdict," Bulgarian Justice Minister Anton Stankov said on Saturday.
A court in the northern Libyan city of Benghazi on Thursday had sentenced the five nurses and a Palestinian doctor to death by firing squad for spreading AIDS in a children's hospital.
"We agree with neither the death sentences handed down nor the so-called guilt of the nurses," Stankov said.
The accused were convicted of having deliberately infected more than 400 children with the HIV virus that can lead to AIDS by injecting them with tainted blood products at the pediatric hospital in Benghazi.
Forty-three of the children have since died.
All defendants pleaded not guilty to the charges when the trial opened four years ago and the verdicts were postponed several times.
Two of the Bulgarian nurses and the Palestinian doctor said during the trial they were tortured into making confessions.
Libyan leader Muammar al-Qadhafi had said in 2001 the case might be one of a CIA or Mossad plot to experiment with the AIDS virus.
The lawyers of the defendants say their clients have been made scapegoats for inadequate sterilisation of instruments at the hospital.
The European Union and the United States both condemned the guilty verdicts. Washington called it "unacceptable."
But a Libyan spokesman said, "The United States has no right to speak of human rights."
Referring to the abuse of prisoners in a US-run jail in Iraq, Hasuna Shaush said: "Before voicing an opinion on the Benghazi verdict, the United States would have done better to apologise for Abu Ghraib.