Washington - The United States had pushed strongly the independence of South Sudan when it became the world's newest country in 2011.
But now, after a year of civil war, Washington is accused of standing by as it descends into bloodshed.
President Barack Obama has issued a flurry of condemnations and Secretary of State John Kerry travelled to Juba to seek peace.
But the United States dragged its feet in imposing sanctions and an arms embargo, while pleas for restraint have fallen on deaf ears.
The leaders of the two warring sides - President Salva Kiir and his former vice president Riek Machar - are inured to US warnings.
In an op-ed piece published Monday in the Washington Post, National Security Adviser Susan Rice and Kerry again urged both sides to “finally end this conflict without further delay.”
Failing that, they wrote, “those who choose the path of continued conflict and destruction will face greater consequences.”
But David Abramowitz, vice president for policy and government relations at Humanity United, a US-based foundation dedicated to building peace, dismissed such threats as “good rhetoric.”
Jok Madut Jok, professor from the Sudd Institute in Juba and Loyola Marymount University in California, told AFP that Washington's “message is not very clear.”
South Sudan was born July 9, 2011 from the partition of Sudan after 25 years pro-independence rebellion but on December 15, 2013 the new country plunged back into war.
The conflict was trigged by rivalry between between President Kiir and former vice-president Machar, respectively from the Dinka and Nuer peoples, the country's two main ethnic communities.
The civil war has left at least 50,000 people dead - some estimates say the figure is double that - and the new country is showing no signs of moving toward peace.
The United States is seen as the country in the best position to impose a solution after spending 30 years as the principle proponent of what would become an independent South Sudan.
It supported the southern rebellion led by John Garang from 1983 until his death in 2005 and by his successors until the independence ceremony held in July 2011 with Rice in attendance.
Billions of dollars of US aid flowed in as the United States backed the new country, both for moral and humanitarian as well as strategic and oil-related reasons.
Out of loyalty to Juba, Washington reacted quickly to the first fighting in December of 2013. Obama sent in about 100 soldiers.
His diplomacy led several east African countries - Ethiopia, Kenya, Uganda - to act as mediators.
In May, visiting Addis Abeba and Juba, Kerry tried to arrange face to face talks between Kiir and Machar. But his peace efforts, those of Sudan's African neighbours and successive ceasefires all failed.
“But then what happened after that? The US high level attention was not as significant between now and then,” Abramowitz said.
Especially in recent months, Washington has seemed to dither on enacting sanctions on senior South Sudan leaders, and not just against second-tier officials.
On Monday the UN Security Council again condemned continued fighting in the country and once again threatened the warring sides with “targeted sanctions.”
For months now, the 15 members of the council, including the United States, have raised the prospect of asset freezes, travel bans and an arms embargo against Kiir and Machar.
But the United Nations has yet to decide on anything firm.
This has been a disappointment for Human Rights Watch, which wrote to the Security Council last week to press for a weapons embargo on both sides.
Mere condemnations are not enough, it said in an open letter.
But according to Abramowitz, “now the US, who has a responsibility at the security council for drafting security council resolutions on South Sudan, has not been able to table a resolution.
“The reason for that is that the (...) draft resolution that would be tabled to include an arms embargo and the US is opposing that.”
Diplomats say an embargo would mainly hurt Kiir and not the rebellion led by Machar.
Abramowitz said he does not think the next US measures will be sanctions against the South Sudanese rivals.
“The government and the opposition don't understand exactly where is the position of the US,” said Jok Madut Jok.
Sapa-AFP