South Sudanese rebel leader Riek Machar should not return to the post of first vice-president, President Barack Obama’s special envoy to Juba told the US Congress on Wednesday.

“It is not for us to tell South Sudan who its leaders should be,” Ambassador Donald Booth cautioned, but added: “Given all that has happened we do not believe it would be wise for Machar to return to his previous position in Juba.”

Dr Machar is currently staying in Khartoum, Sudan, after fleeing what Mr Booth described as President Salva Kiir’s “egregious action of militarily pursuing his first vice-president out of South Sudan.”

The US special envoy went on to present unusually blunt criticisms of South Sudan’s head of state.

Mr Booth's litany of complaints marks a shift in the US stance toward the leadership of a country that Washington had helped achieve independence in 2011.

“President Kiir and those around him bear much of the responsibility for the extent to which the Transitional Government has failed to become the representative body it needs to be,” Mr Booth said in prepared remarks to a US House of Representatives panel.

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Dinka tribe privileges

He cited President Kiir's “unilateral implementation of his 28 states decree from December 2015, stoking grievances in many parts of the country and among various tribes for the way it privileges his own Dinka ethnic group.”

After installing Taban Deng Gai, another opposition figure, as Dr Machar's replacement, President Kiir demoted politicians from the Nuer tribe who are loyal to Dr Machar, Mr Booth recounted.

The president “has facilitated a zero-tolerance policy toward dissent both within the government, from fellow politicians, and without, from civil society and the media,” the special envoy charged.

Mr Booth also accused the Kiir-led army of carrying out human rights violations.

“One particularly upsetting aspect of the current crisis is the conduct of South Sudanese government forces,” he said.

“We continue to receive reports of civilians being targeted, including with brutal sexual violence. Recent reports indicate a new campaign by government commanders to recruit child soldiers.”

In a separate testimony to the same House panel on Wednesday, Ambassador Booth's predecessor as special envoy to Sudan and South Sudan warned that Dr Machar's ouster will not bring about unity in the fractured country.

Pressure on African Union

“Taban Deng does not command the loyalty of all those forces that have been fighting the government of Salva Kiir,” said former Ambassador Princeton Lyman.

“Without broad-based participation in a transitional government,” he added, “conflict will surely continue. Indeed, conflict continues now in several parts of the country.”

Ambassador Booth stepped up US pressure on the African Union to move forward “expeditiously” with creation of a hybrid court that would pass judgment on war crimes committed during South Sudan's nearly three-year-long civil war.

The AU should establish an office of prosecutor and hire staff to administer the envisioned court, he said.

The US is “on the verge” of providing the AU with a $3.3 million grant to help clear away obstacles to putting the court in place, Mr Booth added.

He said the US will support imposition of a United Nations arms embargo on South Sudan if the government does not fulfill its stated commitment to accept deployment of a supplemental UN force of 4,000 troops drawn from East African countries.