Bulgaria donated $56,6-million in Soviet-era debt owned by Libya as its contribution to a deal that led to the release of six medics convicted of infecting Libyan children with HIV.

The European Union (EU) newcomer signed on Monday an agreement to donate the debt, accumulated for arms and technical deliveries, to an international fund set up to provide medical aid and help the families of more than 400 Libyan HIV/AIDS victims.

"The agreement once again proves that Bulgaria is a reliable partner which delivers on its promises," deputy Foreign Minister Feim Chaushev told reporters upon signing.

Chaushev said Tripoli had agreed to the amount of the debt which has not been served in the past 18 years.

The medics who spent eight years in a Libyan jail have maintained their innocence and said they were tortured into confessing.

Tripoli returned the medics to Bulgaria in a deal which included medical help, political ties between the EU and Tripoli, and compensation for the families of the victims.

Their death sentences were commuted to life in prison after Libya paid the victims' families $460-million in a settlement arranged by the International Benghazi fund. The Balkan country's president pardoned the six upon their arrival.

The chairman of the fund, Mark Pierini, said the fund plans to pay back the funds to Libya, as and when donors make resources available.

He refused to disclose the level of funds raised so far, citing contributors' requests for anonymity, but noted that besides the Bulgarian contribution, the EU has donated €11,5-million, and Germany €1,5-million.

"We plan to return the funds paid by Libya to the extent we achieve contributions. It's a voluntary organisation," Pierini told Reuters.

Earlier, the Bulgarian government said 27 donors, including 17 governments, nine private companies and one nongovernmental organisation, had also pledged to contribute to the fund.