GENEVA (AFP) - The United Nations said it was "deeply disturbed" by the death sentences handed down in Libya on Wednesday against a son and eight aides of slain dictator Muammar Gaddafi.

"We had closely monitored the detention and trial and found that international fair trial standards had failed to be met," the UN's High Commissioner for Human Rights (OHCHR) said in a statement.

Gaddafi's son Seif al-Islam and the others were accused of crimes including murder and complicity to incite rape during the 2011 revolution.

The prosecutions were notably marred by "the failure to establish individual criminal responsibility in relation to specific crimes," the OHCHR said.

"There were also serious issues relating to access to lawyers, claims of ill-treatment, and trials conducted in absentia," it added.

Seif al-Islam, who had been Gaddafi's heir apparent, was tried in his absence because he is held by militia in the southwestern hill town of Zintan that is opposed to the Tripoli authorities.

He is also wanted by the International Criminal Court.

Among the others sentenced to death were ex-intelligence boss Abdullah Senussi and Gaddafi's last prime minister Al-Baghdadi al-Mahmudi, but they both have the option of appealing to the supreme court.

The 37 defendants were charged for crimes allegedly committed in the final weeks of Gaddafi's regime.

"It is crucial to ensure accountability for serious human rights violations. However this needs to be done with scrupulous adherence to international fair trial standards...

"Failing this, injustice is only compounded," the UN added, restating its opposition to the death penalty in all circumstances.

Libya has been wracked by conflict since Gaddafi's overthrow, with rival governments and powerful militias battling for control of key cities.

The trial, which opened in the Libyan capital in April last year, has been repeatedly criticised by human rights groups.