"The majority of the victims are women, and in some cases children," the UN peacekeeping mission in South Sudan (UNMISS) said in a statement, calling for "immediate action to safeguard recent gains in the peace process".
The United Nations "is concerned by the recent increase in serious human rights violations allegedly committed by some undisciplined elements within the South Sudanese Army," it added.
Between July 15 and August 20, UN teams reported "alleged violations including one killing, 27 allegations of torture or ill-treatment, such as beatings, and simulated drowning in some cases, 12 rapes, six attempted rapes and eight abductions," it added.
The army launched a disarmament programme following a wave of revenge attacks in late December among rival ethnic groups that left nearly 900 people dead according to UN figures, although others estimated the figure to be far higher.
In the attacks, an estimated 8,000-strong militia force from the Lou Nuer people rampaged through Jonglei`s Pibor county, massacring members of the rival Murle group, abducting women and children, razing villages and stealing cattle.
The response of the army -- made up of former rebel fighters, who became the South`s official army at its independence in 2011 -- has been heavily criticised for its brutal treatment of civilians.
Human Rights Watch has reports of "soldiers shooting at civilians, and ill-treating them by beatings, tying them up with rope, and submerging their heads in water to extract information about the location of weapons".
The group, in an open letter sent to the government on Thursday, said it had "also received credible reports of rape and beatings and additional acts of torture."
Jonglei was one of the areas hardest hit in Sudan`s 1983-2005 north-south civil war, which ended in a peace deal that paved the way for the South`s full independence.
But the new nation is awash with guns, while heavily armed communities that were once pitted against each other during Khartoum`s rule remain rivals.