Tripoli, Jul 28 (CNN) : A Tripoli court sentenced several officials in the former regime of slain Libyan leader Moammar Gadhafi to death Tuesday (July 28) – including Gadhafi's son Saif al-Islam Gadhafi in absentia – drawing criticism from the U.N. human rights office.

Saif al-Islam Gadhafi was ordered to face a firing squad for his role in trying to quash the 2011 revolution that led to his father's ouster.

Several other Gadhafi regime officials were also sentenced to death, including former Libyan spy chief Abdullah al-Senussi, former Prime Minister Al-Mahmoudi al-Baghdadi, and former foreign intelligence chief Buzeid Dorda.

Saif al-Islam Gadhafi was not at the trial; he was last known to be held by a militia group in the northwestern city of Zintan, where he was held after his capture in November 2011.

Since Gadhafi was not at the trial and Libya is politically and militarily fractured – Tripoli is home to one of two rival Libyan central governments – it's not clear what happens to him now.

The Zintan militia, the last group known to have him, doesn't recognize the government and court in Tripoli and had cut off cooperation with the proceedings.

Gadhafi faced charges relating to the 2011 revolution and attempts to suppress the uprising, including the killing of protesters – a crime punishable by death.

U.N. human rights office 'deeply disturbed'

The office of the U.N. High Commissioner for Human Rights said Tuesday it was "deeply disturbed" by the verdicts and sentences, asserting that the trial failed to meet international standards for fair trials.

"Among the key shortcomings are the failure to establish individual criminal responsibility in relation to specific crimes," the U.N. office said. "There were also serious issues relating to access to lawyers, claims of ill-treatment, and trials conducted in absentia.

"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 and with full respect for the rights of the defendants. Failing this, injustice is only compounded."

The office urged Libya to pass legal reforms and ensure that the verdicts could be appealed.

Gadhafi was being held in Zintan during the start of the trial last year and joined three of the 24 trial sessions by video link, but he "has not been seen or heard from since June 2014," Human Rights Watch said Tuesday.

The North African nation has been the scene of chaos and fighting between various groups and militias after the 2011 revolution. Libya now has two central governments, with the internationally recognized one in the east, and the other, self-proclaimed one based in the west in Tripoli.

Human Rights Watch: Gadhafi attorney didn't give final defense

Like the U.N. human rights office, Human Rights Watch asserted that the trial was flawed. Among other things, the group said that it appeared that Gadhafi's court-appointed lawyer didn't present a final defense pleading on Gadhafi's behalf.

It called for Libya's Supreme Court to review the cases.

"This trial has been plagued by persistent, credible allegations of fair trial breaches that warrant independent and impartial judicial review," said Joe Stork, deputy Middle East and North Africa director for Human Rights Watch. "The victims of the serious crimes committed during the 2011 uprising deserve justice, but that can only be delivered through fair and transparent proceedings."

Gadhafi was once considered the heir apparent to his father, who ruled Libya for over 30 years.

Before the 2011 Libyan civil war, he was believed to be a moderate in comparison to his father.

A NATO strike in May 2011 killed his younger brother who had a similar name, Saif al-Arab Gadhafi.