KABUL (Reuters) - Afghan authorities ordered the arrest of nine bodyguards of Vice President Rashid Dostum on Tuesday, over allegations of sexual violence and torture of a political rival in a case that has caused deep alarm among Western allies of the Kabul government.

Dostum has proved a difficult and unpredictable ally for President Ashraf Ghani, who brought the ethnic Uzbek into the national unity government after the disputed 2014 election, despite longstanding accusations of human rights abuses.

Dostum faces allegations of ordering his men to detain Ahmad Ischi, a former ally, who says he was severely beaten and threatened with sexual violence over several days.

Ghani has promised a thorough investigation of the case, which has caused deep unease among allies of the Western-backed government in Kabul, including the United States, which issued a statement last month expressing its concern.

Officials from the attorney general's office confirmed the arrest order had been issued against the men after repeated attempts to question them as part of the investigation.

"We sent several letters telling them to come and answer questions regarding the allegations, but nobody showed up, so the investigating team issued an arrest warrant against nine bodyguards who are directly involved," said Jamshed Rasooli, the spokesman for the attorney general's office.

No comment was immediately available from a spokesman for Dostum.

The case has caused severe embarrassment for Ghani, whose relations with Dostum, a veteran of decades of Afghan politics, have long been under strain.

His standing within the ethnic Uzbek community, estimated to make up about 9 percent of the population, has made it tough for Ghani to replace him, despite the concerns of foreign allies.

(Reporting by Mirwais Harooni; Editing by Clarence Fernandez)