* South Sudan peace talks resume next month
* Conflict brought South Sudan to brink of civil war
* Uganda is member of IGAD, which will monitor ceasefire
By Edmund Blair and Aaron Maasho
ADDIS ABABA, Jan 29 (Reuters) - Uganda should start withdrawing troops from South Sudan, where they have been backing government forces against rebels, to avoid worsening a crisis in the world's newest state, major donor Norway said on Wednesday.
The comments were the clearest statement of concern from a member of the troika of South Sudan's main Western backers about the impact of Uganda's military presence.
Analysts had worried Kampala's intervention risked turning the clashes into a regional conflict.
Ugandan President Yoweri Museveni, who backed South Sudan's ruling SPLM in its long conflict with Sudan before independence in 2011, sent his troops across the border at Juba's invitation shortly after fighting began in mid-December.
Both South Sudan's government and rebels have agreed to a ceasefire and shaky peace talks are due to resume in neighbouring Ethiopia on Feb. 7.
"It is now important that President Museveni of Uganda starts the process of reducing and later pulling out the Ugandan troops that are in Juba and surrounding areas," Norwegian Foreign Minister Borge Brende told Reuters.
"I also appeal to President (Omar Hassan) al-Bashir of Sudan to refrain from any kind of intervention," he said, noting that Sudan - which frets about South Sudanese oil fields from which it derives revenues via pipeline fees - had so far stayed out.
Brende said he had delivered his message to Uganda's foreign minister on the sidelines of an African Union meeting in Addis Ababa, and to Sudan on a trip to Khartoum.
Britain, which with Norway and the United States has been one of South Sudan's main Western sponsors, said Ugandan forces should act defensively and "de-escalate" the conflict, but has not specifically called for the troops to start leaving.
Fighting broke out between rival groups in the presidential guard in South Sudan's capital Juba and quickly spread to oil-producing areas, largely along ethnic lines.
South Sudan President Salva Kiir accused Riek Machar, the vice president he sacked in July, of launching a coup.
Machar, who is now in hiding, dismissed the allegation, saying Kiir had taken advantage of an outbreak of fighting to round up political rivals.
Uganda's government initially said its troops were sent in to help stranded Ugandans and to secure Juba's airport and Kiir's official palace.
Rebels, led by Machar, accused Uganda of launching air strikes and other assaults against their positions. After initial denials, Uganda acknowledged taking an active role in the fighting to help government forces.
"He was requested by Salva Kiir in a difficult moment to contribute with this support," the Norwegian minister said of Museveni's actions. "There is now a new political situation because there is an agreement on the cessation of hostilities."
"We also have to make sure that all the neighbouring countries play a very constructive role and refrain from any kind of action that can add to the crisis," he said.
Uganda is a member of IGAD, the regional grouping sponsoring peace talks in Addis Ababa that led to a cessation of hostilities deal this month. Sporadic fighting has persisted.
Ethiopia, which currently chairs IGAD, earlier voiced concerns about Uganda's intervention. Others in the grouping, which aims to set up a ceasefire monitoring mechanism, have played down worries about Uganda's role.
Brende said Kiir and Machar must authorise negotiators at talks to reach a political deal and ensure aid reaches isolated communities and the more than half a million displaced people.
(Writing by Edmund Blair; Editing by Richard Lough and Andrew Heavens)