JUBA - The world's youngest nation, South Sudan, celebrated its second anniversary on Tuesday.
Independence celebrations come amid simmering tensions with its neighbour, Sudan.
In his address to the nation, President Salva Kiir stressed that one of the biggest challenges facing the fledgling state is stamping out corruption.
Human rights campaigners agreed and warned that South Sudan was guilty of widespread human rights abuses.
US activists who backed the split from Sudan claimed in a letter that the fledgling country faces "an increasingly perilous state".
Signatories of the letter, which accuses the new country of failing its own people and repeating the mistakes of previous rulers before independence, include John Prendergast, a former director for African affairs at the White House's National Security Council.
"Many people in South Sudan are suffering, yet government officials seem to care only about themselves," reads the letter, also signed by former US State Department officials including Ted Dagne, a former advisor to the government.
"We joined you in your fight against these very abuses by the Khartoum regime for many years," they wrote in the letter, which is addressed to South Sudan's President Salva Kiir.
"We cannot turn a blind eye when yesterday's victims become today's perpetrators," they said.
The activists, who call themselves the "Friends of South Sudan", also said there was "clear evidence of massive corruption" and urged "profound reform".
South Sudan split from Sudan on July 9, 2011, after its people voted overwhelmingly for independence in a referendum six months earlier, part of a 2005 peace deal that ended one of Africa's longest civil wars.
The oil-rich but grossly impoverished region -- wracked by war for almost five decades -- was born as one of the least developed nations in the world.
Kiir has suspended several key officials over corruption allegations and launched national reconciliation efforts, but the activists were scathing as to the state of the nation.
"After almost nine years of self-rule, the government is still failing to meet the basic needs of its people," the letter added.
"Despite claims that vast sums have been expended on investment in infrastructure, there is very little to show in the way of roads, medical services, and education for millions of South Sudanese who greeted the prospect of independence with eagerness and hope."
Tensions with former arch-foes in Sudan remain, with the two nations clashing in brutal border skirmishes last year following bitter arguments over oil.
South Sudan has also been battling several rebel militia forces, some of whom Juba accuses of being backed by Khartoum, others the legacy of years of civil war.
But the activists accused Juba of targeting groups based on their ethnicity or because they oppose the current leadership.
"Over the past several years -- but the last six months in particular --South Sudan government security forces have engaged in a campaign of violence again civilians simply because they belonged to a different ethnic group or they are viewed as opponents of the current government," the letter read.
"This violence is shocking and has included rape, murder, theft and destruction of property."