Al-Islam, son of Muammar Gaddafi, Libya's ruler, has been a frequent participant in talks between the EU and the families of the infected children.
The six foreign medics - five Bulgarian nurses and a Palestinian doctor - were convicted in December of deliberately infecting 426 children with the HIV virus.
The highly politicised trial has hampered attempts by Libya, a member of Opec, to restore full relations with the West.
The medics say they are innocent and were tortured to make them confess, and the US and the EU have stepped up pressure on Tripoli to release them.
Al-Islam said: "Those who have the decision to end this crisis are two: the Libyan judiciary system and the Libyan families."
Ferrero-Waldner and Steinmeier flew to Libya earlier on Sunday to try to advance efforts to solve the case.
They also visited some of the infected children at a Benghazi hospital and met some of the families.
'Negligence' at fault
Critics of the trial have said negligence and poor hospital hygiene are the real culprits and the six medics are scapegoats.
|Saif al-Islam, second right, said the|
political crisis is in its 'last mile' [AFP]
Libya has suggested it can free the nurses if an agreement is reached to pay compensation to the families of the children.
Tripoli has demanded 10m euros for each infected child's family. Bulgaria and its allies have rejected this, saying it would be an admission of guilt, but have offered a fund for treatment for the children at European hospitals.
Driss Lagha, chairman of the Association for the Families of the HIV-infected Children, quoted Steinmeier and Ferraro-Waldner as saying that, during their Benghazi visit, they were hoping for a speedier resolution of the crisis.
Lagha said: "We also discussed the issue of compensation, but this is still under negotiation."
He has said previously that he hoped to reach an agreement on the case before June 21.
Lagha is the father of one of the hundreds of Libyan children infected and is one of the Libyan negotiators seeking a settlement of the case.