The UN Security Council on Tuesday requested a special report about weapons flows to South Sudan after major powers failed to agree on imposing an arms embargo on the war-torn country.

The 15-member council unanimously adopted a US-drafted resolution that renews sanctions on South Sudan for a year and tasks a panel of experts with preparing the detailed report within three months.

The government and rebels continue to stockpile weapons despite a peace accord signed in August to end a brutal war, UN experts have told the council.

US Deputy Ambassador David Pressman said the continued arms flows pose a "serious threat to the success of the peace agreement and the stability of the region."

The report will focus on arms flows since a new government of national unity was formed last month and provide an analysis of security threats to the new authorities.

Russia, China and Egypt oppose a proposed arms embargo that the United States and Britain argue would help end the conflict that has engulfed South Sudan since December 2013.

Seeking to shore up the peace accord, the council piled pressure on rebel leader Riek Machar to return to Juba last month to form a transitional unity government with President Salva Kiir.

Threats of additional measures including possibly an arms embargo were removed from the draft resolution following objections from Russia, China and Egypt, diplomats said.

"Renewing sanctions and expanding them is not the ideal manner to address conflicts at this stage," Egyptian Ambassador Amr Aboulatta said, adding that there had been a rapprochement of the parties.

South Sudan's Deputy UN Ambassador Joseph Moum Malok told the council that the resolution questioned his country's "right to arm" and "protect itself against any aggression from within or external."

The war in South Sudan began when Kiir accused Machar of plotting a coup.

The conflict has torn open ethnic divisions, shocking the world by the scale of its atrocities, including gang rapes, wholesale burning of villages and cannibalism.

The violence has killed tens of thousands of people and driven more than two million from their homes since it erupted just two years after South Sudan won independence.