BEIRUT, Lebanon, July 5 (UPI) -- At least nine inmates were killed at a Syrian prison Saturday when guards fired during a riot by Islamist prisoners, Human Rights Watch said.

Human Rights Watch officials based in Lebanon reported receiving cell phone calls from prisoners at Saydnaya Prison 20 miles from Damascus, The New York Times reported. The Syrian government had not made any official announcement of trouble at the prison.

Nadim Houry, a Human Rights Watch researcher in Syria and Lebanon, called the prison, which houses 1,500 inmates, "a symbol of repression in Syria."

"It's the place where everyone tried by the state security court ends up serving their sentence, and inmates are often tortured and ill-treated there," he said.

Houry said the trouble began when a group of military police came to the prison early Saturday, planning a security sweep. They reportedly fired at Islamist prisoners protesting bad treatment.

The prisoners then took guards and prison administrators hostage, demanded the police drop their weapons and took them hostage when they did.