UN leader Ban Ki-moon has condemned harassment and threats against UN workers by South Sudan's army and police amid heightened concern about the world's newest nation.
Ban complained in a report to the UN Security Council of 67 cases of "harassment, threats, physical assault, arrest and detention of United Nations staff" and seizure of UN vehicles between May 7 and November 5.
The US government took up the complaints in an official approach to South Sudanese diplomats in New York on Friday, diplomatic sources said. The UN Security Council is to discuss South Sudan on Monday.
Ban said most of the cases involved the Sudan People's Liberation Army (SPLA), which has already been involved in shooting down a UN helicopter, and the national police.
On October 19, a woman UN staff member was "severely beaten" in the capital, Juba, and then detained for an hour following "a traffic encounter with a military pickup carrying armed uniformed men."
Diplomats said the case involved a convoy of South Sudan's President Salva Kiir.
From October 25 to November 3, another five cases of "harassment and extortion" of UN staff by South Sudanese security personnel were reported in Juba, Ban added.
Outside Juba, about 50 SPLA soldiers tried to force their way onto a helicopter belonging to the UN Mission in South Sudan (UNMISS) in troubled Jonglei state on October 26, the report said.
The helicopter was taking SPLA medical team to investigate reports of SPLA soldiers falling sick. "The UNMISS crew was forced to let 16 SPLA soldiers on-board for evacuation to Juba," Ban said.
The UN normally has a status of forces agreement with countries where it operates.
Ban said: "Continuing violations of the status of forces agreement between the United Nations and South Sudan by elements of the security sector remain a serious concern.
"I call upon the government to urgently take steps to prevent any recurrences and bring to account those responsible for violations, including the incident in Juba on October 19," the UN leader said.
South Sudan's UN mission did not immediately respond to queries. The incidents are, however, part of a long line of attacks and detentions involving UN workers since South Sudan became independent in July 2011.
South Sudan expelled a UN human rights investigator in October 2012 accusing her of writing "false" reports about army abuses.
Two months later, South Sudan's army shot down an UNMISS helicopter killing all four Russian crew on board.
South Sudan's military claimed at the time it had not been told the UN flight would be in the region of Jonglei. But a national investigation into the incident has still not been completed.
Previous UN reports have outlined arrests, thefts of UN vehicles and complained of "ill-discipline" among South Sudanese troops when handling UN workers.
UNMISS has 7,600 troops and police in South Sudan, which broke away from Sudan after a two-decade-long civil war up to 2005 in which two million people died.
It has oil reserves but remains one of the world's most impoverished countries with barely any usable roads outside Juba.
Ban's report highlighted concerns over human rights abuses by the military and security forces in Jonglei state where hundreds have died in unrest over the past two years. Salva Kiir's government has also been hit by corruption scandals, which led him to sack his finance minister in June, and political tensions.
But Ban said there was cause for "cautious optimism" over South Sudan's future after the government vowed to improve its performance.