Only six years ago the Sudan People's Liberation Army was a ragtag group of guerrilla fighters battling a bloody civil war with Sudan's north. Next weekend, when the south breaks away and becomes the world's newest country, the SPLA becomes a national army.

The U.S. is investing tens of millions of dollars into this fledgling military, one that is massing troops on the internal north-south border as tensions -- and violence -- with the north rise. SPLA troops are battling rebel militias in hotspots around the south, and fears of renewed war with the north remain high.

But international rights groups say those soldiers have been responsible for human rights abuses, including killings.

U.S. Sen. Patrick Leahy, who sponsored a law that prohibits the U.S. from giving assistance to foreign military units that violate human rights, says he is concerned about reports of abuses.

The State Department is giving nearly $100 million in yearly assistance to train and support the SPLA, and it says it is monitoring the behaviour of the former guerrilla fighters.

But monitoring the 140,000-plus-member army of a developing nation the size of Texas is a nearly impossible task, opening the way for abuses.

In April, a 700-member battalion of SPLA Commandos -- the most highly trained of the SPLA's fighters -- fired indiscriminately on unarmed men, women and children during an attack on a rival ethnic group at a remote Nile River village in Jonglei state, killing or wounding hundreds of civilians, according to witness accounts in a confidential U.N. report.

After an inquiry from Congress, the State Department investigated and found that no U.S. assistance is being given to the two commanders named in the U.N. report or to the commando unit as a whole. The State Department said it would exclude those involved from receiving future assistance until an investigation proves they were not involved in violations.

"The Leahy Law serves a vital purpose in seeking to ensure that U.S. aid does not go to foreign military and police forces who commit heinous crimes," Leahy, a Vermont Democrat, told The Associated Press. "I am concerned with the reports of abuses by Southern Sudanese troops and expect the law to be applied vigorously and consistently."

Human Rights Watch and Amnesty International on Thursday released a report that urged the new southern government to prosecute and prevent abuses by southern security forces.

The report noted that since the south's independence vote in January, "soldiers have been responsible for grave human rights abuses, including unlawful killings of civilians and looting and destruction of civilian property."

"The government needs to demonstrate its commitment to combat a growing culture of impunity for abuses by its security forces," Daniel Bekele, director of the Africa division of Human Rights Watch said. "It should make sure that rank-and-file soldiers and their officers, as well as the police service, know and understand their obligations, and are held accountable for violations."

Since Sudan's decades-long civil war ended in 2005 -- a war in which some 2 million people died -- the U.S. government has given more money than any other to programs aimed at professionalizing the SPLA. According to research by the Open Society Foundations, the Obama administration is requesting nearly $160 million in assistance to the armed forces in Southern Sudan for fiscal year 2012.

Southern Sudan becomes a new country on July 9.

Sudan experts say a responsible and professional southern army will be essential to improving security in the vast and underdeveloped south, where basic principles of rule of law and justice have yet to be upheld and enforced by southern security forces.

Violence is high in the south already. According to the U.N.'s latest statistics, local conflicts such as cattle-raiding and battles between rebel militias and the SPLA have claimed more than 1,800 lives this year.

The U.S. assistance is to help the SPLA develop logistics, engineering abilities, medical, and command and control abilities. A State Department official who spoke on the condition of anonymity told AP by email that the support is intended to "transform" the SPLA "from a guerrilla force into a standing army under civilian control and respectful of international humanitarian law."

The official, who was not allowed to be quoted according to State Department rules, said that U.S.-funded training "includes a component on respect for human rights and respect for rule of law."

The official confirmed that a current State Department contract to PAE, an American military contractor, provides provisions, salaries, and limited logistical support for Ethiopian forces who are training southern forces in a remote army training camp called New Kush, nestled in the Imatong Mountains on Southern Sudan's border with Uganda.

New Kush has proven to be a problem for U.N. human rights investigators, who have sought access to the camp for more than two years to follow up on allegations by community members living near the training camp that southern troops had taken young women from the nearby village as "wives" and that rape and other abuses were occurring inside the site.

Although U.N. staff from the mission's child protection unit were allowed to visit New Kush in February 2010 as part of a delegation led by the southern army's own child protection division, no independent investigations of the human rights conditions inside the camp have been conducted since the U.N. mission was established after the civil war ended in 2005.

Since 2006, the New Kush camp has been used for training the SPLA's special forces -- or Commando units, the same forces involved in the Jonglei civilian deaths. Both international trainers, Western contractors and consultants, and Ethiopian troops -- all funded through State Department programs -- have worked at New Kush.

The State Department official told the AP on Thursday that the "commando training conducted by the Ethiopians focuses on professional military tactics and specialized skills."

But the SPLA has a lot of growing up to do as the world's youngest national military. In a report last November by the Small Arms Survey and authored by Richard Rands, whose own British company Burton Rands previously was subcontracted through U.S.-funded PAE contracts, the author concluded that an "overarching strategy" for the long-term transformation of the SPLA from a guerrilla movement to a conventional army "has not yet emerged."

Rands wrote that "diplomatic pressure and international support and advice" will be needed to urge the army to conduct its own strategic review after independence and then to develop a "coherent defence strategy."