South Sudan's army has set up a probe into the killing of "innocent people", the government said Thursday, in the wake of grim reports of ethnic slaughter and atrocities.

The army has formed a committee "to investigate those involved in killing innocent people" the government said in a statement.

The announcement follows a statement by the United Nations peacekeeping mission (UNMISS) that it is "actively collecting information" on reported atrocities to be used for future official investigations.

Thousands of people are feared to have been killed in the fighting, pitting army units loyal to President Salva Kiir against a loose alliance of ethnic militia forces and mutinous army commanders nominally headed by ex-vice president Riek Machar.

UNMISS said this week it was "gravely concerned about mounting evidence of gross violations of international human rights law that have occurred."

It reported "extra-judicial killings of civilians and captured soldiers" and the "discovery of large numbers of bodies" in the capital Juba as well as in the towns of Bor and Malakal.

Human Rights Watch has said South Sudanese soldiers and rebels both executed people based on their ethnicity.

Another army committee will investigate the causes of the "Tiger Division incident", referring to infighting within the special army presidential guard units that sparked the unrest almost three weeks ago, that has taken the young nation to the brink of all-out civil war.

South Sudan's warring parties have sent delegations to Ethiopia for peace talks aimed at ending the conflict, even as the army said its troops were moving again on the key rebel-held town.

On Tuesday, Machar told AFP via satellite phone that he was not yet ready to agree to an immediate ceasefire nor hold face-to-face talks with Kiir.

But the government said it was committed to peace.

"We are participating in talks because we want peace for our people even though the rebel groups have not accepted a cessation of hostilities," the statement added.