A “glaring gap” exists in the world’s ability to investigate targeted state killings of human rights defenders and journalists such as Jamal Khashoggi, said Amal Clooney, the UK special envoy on media freedom.

She also said the UN special rapporteur Agnès Callamard, who undertook the UN’s investigation into Khashoggi’s murder, “had been forced heroically to manage a large-scale investigation with ridiculously few resources”.

Clooney said she expected a specialist legal panel, set up by the UK government and due to report soon, to champion a new standing UN investigatory mechanism into such killings.

Callamard has herself called for such a mechanism, saying there “a large accountability deficit”.

Callamard’s report into Khashoggi’s death, published in June, accused Saudi Arabia of premeditated murder on 2 October 2018, but she received no cooperation from Saudi Arabia in compiling her document. She found “every expert interviewed said it was inconceivable that an operation on this scale had been carried out without the Saudi crown prince, Mohammed bin Salman, being aware at a minimum of some sort of mission of a criminal nature directed at Khashoggi was being launched”.

Speaking as the anniversary of Khashoggi’s murder approached, Callamard said her proposal for a standing UN investigatory mechanism was “meeting resistance from within the UN”, largely from leaders determined to defend national sovereignty.

She urged world leaders to speak out about media freedom, claiming too many leaders were “instrumentalising” attacks on the press. She said she wanted to stage a UN session on media freedom at the G20 leaders summit in Riyadh next year.

In a CBS 60 Minutes interview, timed for the anniversary of the killing, the Saudi crown prince said he took responsibility for the death of Khashoggi in the sense that it had happened on his watch but did not admit any involvement.

He said: “When a crime is committed against a Saudi citizen by officials, working for the Saudi government, as a leader I must take responsibility.” He described the killing as a mistake and a heinous crime. “I must take all actions to avoid such a thing in the future.”

Prince Mohammed insisted that even though officials very close to him were alleged to have been involved he had had no prior knowledge. He insisted if anyone had any relevant evidence they should produce it, but was not asked why he had refused to cooperate with the UN inquiry.

Court proceedings against 11 unidentified individuals started in Riyadh, the Saudi capital, in January. Five are facing the death penalty.

The trial is taking place behind closed doors, but diplomats from the five permanent UN security council members and Turkey are allowed to attend on condition they do not disclose details of the proceedings. Turkey has been invited since Khashoggi was murdered in the Saudi consulate in Istanbul, and Ankara provided much of the evidence including tapes that show the complicity of Saudi government officials.

Callamard, speaking at the UN, said: “I have discovered as I conducted my investigation into Khashoggi that the UN has no meaningful role to play or had failed to act in in a meaningful fashion.”

She said the proposed UN standing mechanism could act either in support of national actors undertaking investigations of targeted killings or instead establish an international inquiry.

She chastised world leaders including Donald Trump and Vladimir Putin for using summits such as the recent G20 to greet Mohammed bin Salman warmly, so normalising attacks on media freedom.

She said: “World leaders have a duty to speak up against those that denigrate press freedom. I am not suggesting that they stop diplomatic relations, I am simply asking them to stand up or simply walk out when there is such a display of the violation of the values the UN stands for.”

Clooney said: “I totally agree there is a glaring gap in the international system of protection when it comes to establishing facts in a cross-section of situations that require proper investigation.”

The proposal for a UN investigatory body is one of the five priorities she plans to highlight when she gives a report to the UK government on media freedom.