The medics, sentenced to death by firing squad last year for deliberately infecting more than 400 children at a Benghazi hospital, insist they are innocent and that the only evidence against them were confessions extracted under torture.
"After a hearing today, the High Court postponed its final decision to 31 May when it will rule whether to uphold the death penalty on the six defendants or order another trial," a court official said in Tripoli on Tuesday.
At least 40 of the 426 infected children have died of Aids, increasing widespread outrage in Libya over the case.
The United States and the European Union have slammed the verdicts, which have impeded Tripoli's efforts to emerge from decades of diplomatic isolation and renew ties with the West.
The Bulgarians and Palestinian
doctor are contesting the ruling
Aids experts testified to a lower court last year that the epidemic started before the nurses arrived at the hospital, possibly due to poor hygiene or the unsafe use of syringes and blood products.
Tripoli has said that if Sofia pays financial damages to the victims' families, builds a modern hospital in Libya, and also provides medical treatment in Europe, it might release the nurses.
But Sofia has rejected financial compensation because it says the nurses are not guilty.
The nurses, who have been imprisoned since 1999, insist Tripoli has made them scapegoats to hide the poor state of the hospital and deflect guilt from its Libyan management.
Last week, Libyan leader Muammar al-Qadhafi rejected calls from the West to release them, saying: "They (the West) consider our people sheep".