(Recasts with deployment of regional force, changes dateline)

By Aaron Maasho

ADDIS ABABA Aug 5 (Reuters) - South Sudan's government on Friday agreed to allow a regional protection force to be deployed in the country, a regional development group said, following fighting between rival groups last month in the country's capital, Juba.

Such a force has been a key demand of former vice president Riek Machar, the leader of one of the factions involved in last month's fighting.

The agreement was reached at a summit meeting in Ethiopia of the leaders of the eight countries in the group, the Intergovernmental Authority on Development.

"The government of South Sudan has accepted (the deployment of troops) with no condition," Mahboub Maalim, IGAD's executive secretary, told reporters after the meeting.

Fighting broke out in July between forces loyal to President Salva Kiir and to Machar, his long-time rival, who had been serving as vice president but who left Juba after the fighting. He has since been replaced as vice president.

Machar had also served as vice president from 2011, when South Sudan gained independence, until he was sacked by Kiir in 2013. After two years of fighting between his forces and Kiir's, Machar returned to Juba in April and resumed the position of vice president as part of a peace agreement.

He left again after the new clashes broke out, killing at least 272 people. He said he would return only if an intervention force was deployed to separate his forces and those of Kiir.

Politics in South Sudan have long been plagued by splits and rivalries as leaders switch allegiances in a complex contest for power and influence in the oil-producing nation, which gained independence from Sudan only five years ago.

Kiir sacked six ministers allied to Machar earlier this week, replacing them with people allied to the new vice president, Taban Deng Gai. That further widened the political division in South Sudan and threatened to lead to more violence .

IGAD's Maalim said Deng Gai, who attended the meeting in Ethiopia, had said he could step down for Machar if he returns to Juba. Deng Gai was not immediately available to comment.

Maalim said military chiefs from member states would travel to Juba, to work with the government there on the deployment of the new force, without giving more details.

Earlier in the day, South Sudan rejected accusations from the U.N. that its soldiers had raped and killed civilians during last month's clashes.

On Thursday, Zeid Ra'ad Al Hussein, U.N. High Commissioner for Human Rights, said at least 217 cases of sexual violence in Juba had been documented during the period of July 8 to July 25 .

Lul Ruai Koang, the government's military spokesman, said the U.N., through the peacekeeping UNMISS force, should provide evidence that government soldiers were involved in the crimes, adding that they had not received any formal complaints. (Additional reporting by Denis Dumo in Juba; Writing by Duncan Miriri; Editing by Larry King)