GENEVA (Reuters) - The outlawed Lord’s Resistance Army (LRA) has stepped up attacks in Democratic Republic of Congo close to the South Sudanese border as a U.S.-supported regional task force pulls out, the U.N. humanitarian office said in a report on Friday.

Forty rebels from the group, which is led by Joseph Kony, kidnapped 61 civilians in a June 7 raid in the Tanganyika mining area near the Garamba National Park in Haut-Uele province, the report said, citing local civil society and aid workers.

The civilians were released after being forced to move goods and food looted by the LRA, and an unknown number of villagers subsequently fled to the nearby town of Gangala Nabodio.

There had been no LRA-related displacement for more than five years in the province, the U.N. said. But aid workers were now worried about the safety of people across a vast area.

“Since the end of the mission of the Regional Task Force (RTF), which was mandated to eliminate the LRA, the security situation has seriously deteriorated in the Garamba National Park,” the U.N. report said.

One international non-governmental organization involved in protecting civilians had canceled its missions this week due to insecurity, it said.

The U.N. has reported a surge this year in LRA abductions of girls and boys around the ages of 12 or 13, as well as elephant poaching in Garamba National Park. A clash with Congolese armed forces in March killed one army officer and three LRA fighters.

Kony’s rebels battled Ugandan forces for about two decades, becoming notorious for their brutality and for kidnapping children for use as fighters and sex slaves.

In about 2005, they were ejected from bases in northern Uganda and what is now South Sudan, and retreated to an area of jungle straddling the borders of South Sudan, Congo and Central African Republic, where the task force continued to track them.

The United States said in March it was switching to a broader African security mission and removing U.S. special forces specifically focused on fighting the LRA, declaring “success” in reducing the group to under 100 active members.

The African Union asked the United Nations to take on the hunt for the LRA and the U.N. Special Representative for Central Africa, Francois Lounceny Fall, has said the LRA may thrive after the U.S.-backed force leaves.

“I am concerned about the impact of this withdrawal as it will create a security vacuum that may be exploited by the LRA and other armed groups operating in the region,” he told the U.N. Security Council this week.

Uganda’s military began withdrawing from Central African Republic in April, saying it had accomplished its mission - although Kony, who has been indicted by the Hague-based International Criminal Court, remained at large.