"Our bottom line on this is we would like to see these nurses and medics allowed to return home," said State Department spokesman Sean McCormack. "The events which have led to their being imprisoned for several years are tragic."
A court in Tripoli adjourned the retrial of the nurses and a Palestinian doctor until June 13 after just the briefest of hearings attended by several European diplomats.
It was the latest delay in the case, which has drawn world headlines.
The prolonged incarceration has been a subject of controversy at a time when Libya seeks to reintegrate with the international community after years in the cold.
The six defendants were originally sentenced to death but dramatically won a reprieve in a supreme court ruling on Christmas Day.
Humanitarian tragedy
Mr McCormack called the death of 51 children a "humanitarian tragedy" and said the Americans and others were working to address some of the problems highlighted by the case.
"All of that said, our bottom-line position is that we believe that these nurses and medics should be allowed to return home," he said.
Asked if he thought the trial was not free and fair, Mr McCormack said: "There have been on some parts some concern about the trial in the past. (I'm) going to leave that behind at the moment."
Presiding judge Mahmud Huwaissa wound up the session by saying the trial was being adjourned for procedural reasons. The accused appeared to be in good health.
Prosecutors also rejected a request by defence lawyer Othman al-Bizanti for the Bulgarians to be released on bail and the judge ordered the six to stay in detention ahead of the next hearing.
But Mr Bizanti expressed optimism after the hearing. "It is a good start. The accused will get justice. What happened (today) was within the framework of the law."
Bulgaria however warned against any further delays. "We hope that the court hearings will be scheduled without prolonged delays," foreign ministry spokesman Dimitar Tsanchev said.
A French foreign ministry spokesman, Jean-Baptiste Mattei, echoed his comments, saying "we hope this new trial will lead to a quick decision... for the liberation of the Bulgarian nurses and the Palestinian doctor without delay."
US Secretary of State Condoleezza Rice, on a visit to Bulgaria last month, said the nurses had spent "too long in captivity" and voiced hope they would soon be freed.
The nurses and the doctor were convicted on May 6, 2004 by a court in Benghazi, north eastern Libya, on charges of having knowingly injected at least 426 children with AIDS-contaminated blood at a local hospital.
Fifty-one of the children have since died.
Terrible crime
Forty members of the victims' families were protesting outside the court, brandishing placards and photos of the sick children and blocking the road. "It is a terrible crime," they yelled.
In the search for a deal, Sofia and Tripoli have set up a fund to help fight AIDS in Libya and support the families of the infected children.
The fund is seen as a compromise between Libya's demands for compensation and Bulgaria's refusal to pay anything other than "humanitarian aid" on the basis that the nurses were innocent.
"All the sides are working to close this issue in a manner that is acceptable for both the parties. We hope that this affair will be definitively over before the end of the year," said Salah Abdessalam, director of the Kadhafi foundation.
At their initial trial, co-discoverer of the AIDS virus, Luc Montagnier, and Italian professor Vitorio Colizzi said the deadly disease had spread before the nurses' arrival in Libya and was due to poor hygiene in the Benghazi hospital.
But the families have been demanding 10 million dollars for each of the 426 contaminated children, according to Bulgarian television.
The figure would amount to the same compensation paid by Libya to the families of 270 victims of the Pan Am plane bombing over the Scottish village of Lockerbie in 1988.
Zorka Anackova, the mother of one of the nurses, described the latest adjournment as "the most convenient for the Libyans who know (the nurses) are innocent but cannot find a way out, so as not to tell that to the families" of the infected children."