UNITED NATIONS: The United Nations has moved hundreds of peacekeepers to a troubled state in South Sudan ahead of an expected government offensive against a rebel group, a UN envoy said.

The deployment comes as UN leader Ban Ki-moon expressed concern about extrajudicial killings by the South Sudan army and its "reprehensible" downing of a UN helicopter in December, in a report to be discussed by the UN Security Council on Tuesday.

The UN Mission in South Sudan (UNMISS) has moved the peacekeepers into Jonglei state, the scene of large-scale massacres ahead of a government operation against fighters loyal to rebel leader David Yau Yau.

"We are expecting a military operation against David Yau Yau to happen quite soon," UNMISS chief Hilde Johnson said.

The former Norwegian government minister raised fears that civilians could be caught up in the offensive.

"If the military operations start, as we think they might quite soon, we will have to be present on the ground to protect civilians to the maximum extent possible," she said at the International Peace Institute on Monday.

UNMISS has a mandate to protect civilians in South Sudan but does not carry out operations with government forces against rebel forces. "If the perpetrator also is the government's own army we have a challenge on our hands," she said.

The UN has about 5,000 peacekeepers in South Sudan but has had to move available troops to Jonglei because of the new emergency, and Johnson said the mission could not cope with a new crisis in the huge, remote country.

South Sudan split from Sudan in July 2011 after more than two decades of civil war which left two million dead. But on top of lingering tensions with its arch-rival neighbor, the South Sudan government has had to confront ethnic tensions and rebellions of its own.

After being ravaged by war, South Sudan is struggling to stem rising insecurity and transform a rag-tag ex-rebel army into an effective army and police force.

Yau Yau, an ethnic Murle based in Jonglei, launched an uprising last year after losing an election. He pulled out of a government-sponsored disarmament of militant groups while tensions have also risen with the rival Lou Nuer tribe.

Estimates of Yau Yau's following range from several hundred to 5,000 fighters. South Sudan has repeatedly accused Sudan of supplying weapons to Yau Yau, whom the government also blamed for a cattle raid that killed over 100 Lou Nuer in February. The Khartoum government denies providing weapons to Yau Yau.

Jonglei has been the scene of repeated massacres between the Murle and the Lou Nuer, including one in December 2011 when a column of about 8,000 Lou Nuer killed at least 600 from the rival tribe.

Johnson said the UN and local leaders had prevented a similar massacre in the same state on January 28 when a column of at least 6,000 fighters was detected.

The tangled divisions have been complicated by accusations by rights groups that the army has committed killings and other abuses against civilians in Jonglei as it disarms opposition groups.

The United Nations is still investigating the shooting down of one of its helicopters on December 21 by government forces. The four Russian crew members died.

Ban, in a report to the Security Council, called the helicopter attack "deplorable" and "reprehensible". The government said the Sudan People's Liberation Army (SPLA) had believed the helicopter was a Sudanese aircraft delivering weapons to Yau Yau.

The report said restrictions on helicopter movements because of the incident had made "aerial reconnaissance for early warning purposes impossible."

Ban also raised concerns about "extrajudicial killings" and other abuses by the army during its clampdown on militants. These "give rise to serious concerns about the dangers facing civilians during SPLA operations," said the report.

An UNMISS human rights investigator was expelled by the government in October.