(Adds U.S. comment)
By Elias Biryabarema
KAMPALA, July 21 (Reuters) - A South Sudanese rebel delegation was due to hold talks with Ugandan President Yoweri Museveni in Kampala on Tuesday and will ask him to withdraw troops from South Sudan, where they were deployed to help the Juba government, the rebel camp said.
Museveni said in January his forces were supporting South Sudanese President Salva Kiir against rebels led by the former deputy president Riek Machar.
The presence of Ugandan troops has drawn sharp criticism from rebels. South Sudan's other neighbours and Western powers worry that it complicates efforts to end a conflict that erupted seven months ago and which has driven the nation towards famine.
A Ugandan official said the timing of any withdrawal would be decided by Uganda alone and not "dictated" by others.
"The topic is to open a new chapter with Uganda and President Museveni," Miyong Kuon, a spokesman for Machar, said of the rebel delegation. He was speaking in the Ethiopian capital, where peace talks have been held.
Ugandan Foreign Ministry spokesman Fred Opolot would not give details of the agenda for talks but said: "It will generally be around finding a way out of the impasse in the peace process."
The warring factions have held months of peace talks in Addis Ababa since fighting erupted in mid-December, but there has been little progress.
Two cease-fire deals, one reached in January and another in May, have crumbled. Kiir and Machar, long-time political rivals, also agreed in May to start talks on a transitional government but that idea has not advanced since then.
Asked if a withdrawal of Ugandan forces would be discussed with Museveni, a rebel official said: "That is part of the package. How, when and where is something we will discuss."
Uganda's junior foreign minister, Okello Oryem, said the issue was not up for discussion. "Nobody will dictate to us when to leave South Sudan, period," he told Reuters.
Museveni has long had close ties with Kiir, leader of South Sudan's dominant SPLA, which led the fight for more than two decades against Sudan's government in Khartoum before independence. A peace deal in 2005 led to South Sudan's secession in 2011.
Oil producer South Sudan is a major export market for Uganda, and trade has been hurt by the eruption of fighting.
The United States, meanwhile, condemned ground attacks by rebels against government soldiers in the northern town of Nasir on Sunday and warned that Washington would impose sanctions on those who threatened peace efforts.
The United States has already imposed sanctions on commanders from both sides for violating a cease-fire signed in January.
State Department spokeswoman Marie Harf called on Kiir and Machar to "reaffirm their commitment" to peace and ensure their forces stop all fighting immediately. (Additional reporting by Aaron Maasho in Addis Ababa and Lesley Wroughton in Washington.; Writing by Edmund Blair; Editing by Louise Ireland and Ken Wills)