President Bush today appealed to Libya to free five Bulgarian nurses who face the death penalty by firing squad.

The five Bulgarian women, plus a male Palestinian doctor, were found guilty in 2004 of deliberately infecting 426 children with HIV at a Libyan hospital.

International experts who investigated the incident reported that the six foreign medics had been made scapegoats for an HIV epidemic that was probably caused by poor hygiene practices long before they arrived in Libya, but public opinion in Libya is adamantly against them. More than 50 children have died since becoming infected.

This morning Mr Bush, who was visiting the Bulgarian capital Sofia, courted popularity by using a press conference with the Bulgarian President, Georgi Parvanov, to add his voice to the international chorus calling for the nurses and doctor to be freed.

"We strongly support the release of the Bulgarian nurses in Libya," said Mr Bush.

"They should be released and they should be allowed to return to their families. We will continue to make clear to Libya that the release of these nurses is a higher priority for your country.

"Our hearts also go out to the children who have been infected by HIV/AIDS. Together with the EU, the United States is contributing to a fund providing assistance to the Libyan children suffering from this disease and to their families."

Senior officials from the European Union were in Libya today, visiting the imprisoned medics as part of the patient diplomatic process trying to secure their release.

Seif al-Islam, a son of Colonel Muammar Gaddafi, the Libyan leader, said that the talks between the families and the EU delegation had been "positive".

"If the family's demands are met I think the road is open for a settlement," said Mr Al-Islam, the head of the Gaddafi foundation which has been involved in the negotiations.

Libya has suggested it can free the nurses if an agreement is reached to pay compensation to the families. Tripoli has asked for €10 million (£6.78 million) for the family of each infected child - slightly more than the £6 million that Libya is believed to have paid to the families of the 270 people killed in the 1988 Lockerbie bombing.

Such was the storm of emotion whipped up by the HIV epidemic at El-Fath Children's Hospital in Benghazi - the largest documented HIV outbreak at a hospital in history - that Colonel Gaddafi initially blamed it on a plot by the CIA or Mossad.

In 1999 Libya offered to swap the six medics in return for Mohmed Abdelbaset Al-Megrahi, the Libyan who went on to be convicted of the Lockerbie bombing. The offer, soon withdrawn, fuelled suspicions in some quarters that their arrest and trial was being exploited in the diplomatic power play which surrounded Lockerbie and the international sanctions imposed on Libya.

Two of the world's foremost HIV experts, including one of the original discoverers of AIDS, were invited to investigate the causes of the epidemic. Professor Luc Montagnier and Professor Vittorio Colizzi concluded that poor hygiene on the wards had led to the cross-contamination of the patients, causing the outbreak.

But although the scientists gave evidence for the defence at the medics' trial, they were were found guilty after the prosecution introduced a report by Libyan scientists with contrary conclusions, that suggested the children were deliberately injected with tainted blood.

The sentence was appealed and overturned. But the retrial excluded non-Libyan scientific evidence, and resulted in another death sentence in December 2006 for nurses Kristiana Valcheva, Nasya Nenova, Valia Cherveniashka, Valentina Siropulo and Snezhana Dimitrova, and Doctor Ashraf Al-Hajouj.

The five Bulgarian women wept as Judge Mahmoud Haouissa pronounced the sentences at the end of a trial that was condemned by scientists, Western governments and human rights organisations. They have complained of being tortured into making a confession during their long period in custody.

An international fund was set up for the HIV families in 2005. Idriss Lagha, a spokesman for the families, said today that the EU negotiators had promised to help to resolve the case. "However, they have not given a clear response" to requests to increase compensation payments, Mr Lagha added.

Benita Ferrero-Waldner, the EU External Relations Commissioner, who visited some of the bereaved families yesterday, said today: "We do see a window of opportunity for a solution and it is our intention to make sure that it doesn't close before we can use it."

She added that the medics could "rest assured that I will continue to work with all my powers to achieve their liberation, the sooner the better".

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