By Michelle Nichols

UNITED NATIONS Aug 17 (Reuters) - United Nations Secretary-General Ban Ki-moon launched an investigation on Tuesday into accusations peacekeepers in South Sudan failed to respond properly to an attack on a Juba hotel by uniformed men who killed a journalist and raped several civilians.

Ban was "alarmed" by the initial findings of a U.N. fact-finding mission into the attack on the Hotel Terrain on July 11 during an outbreak of fighting between troops loyal to President Salva Kiir and those backing former Vice President Riek Machar.

The secretary-general was "concerned about allegations that UNMISS (the U.N. peacekeeping mission in South Sudan) did not respond appropriately to prevent this and other grave cases of sexual violence committed in Juba," Ban's spokesman said.

Ban has launched an independent special investigation to determine the circumstances surrounding the incidents and evaluate the overall response by the U.N. peacekeeping mission, the spokesman said in a statement.

U.S. Ambassador to the United Nations, Samantha Power, said on Monday: "We are deeply concerned that United Nations peacekeepers were apparently either incapable of or unwilling to respond to calls for help."

The "U.S. embassy in South Sudan responded to distress calls from the compound and urgently contacted South Sudanese government officials, who sent a response force to the site to stop the attack," she said in a statement.

Hundreds of people were killed and the United Nations said government soldiers and security forces executed civilians and gang-raped women and girls during and after last month's fighting. South Sudan rejected the accusations.

"The Secretary-General reiterates his outrage over the acts of violence committed by the SPLA (South Sudanese army) and opposition forces in Juba from 8 to 11 July," said the U.N. spokesman. Ban urged the government to investigate all human rights violations and prosecute those responsible, he said..

Human Rights Watch said on Monday it had uncovered evidence of the cold-blooded execution of civilians by security forces during the fighting. It also found evidence of soldiers raping civilians.

The U.N. Security Council authorized on Friday the deployment of a 4,000-strong protection force to ensure peace in Juba as part of the U.N. mission and threatened an arms embargo if the government did not cooperate.

U.N. peacekeepers have been in the oil-producing country since it gained independence from Sudan in 2011. South Sudan descended into civil war after Kiir dismissed Machar as his deputy. They signed a peace deal in August 2015, but implementation was slow and sporadic fighting continued. (Reporting by Michelle Nichols; Editing by Paul Tait)