BAGHDAD (Reuters) - Dozens of insurgents mounted a sustained attack on Abu Ghraib prison outside Baghdad on Saturday, detonating two suicide car bombs and firing rocket- propelled grenades before U.S. troops repelled the assault.

At least 20 U.S. soldiers and 12 detainees were wounded in the carefully planned attack, which began at around 1500 GMT and lasted for around an hour, the U.S. military said.

"A group of between 40 and 60 insurgents attacked the U.S. forward operating base at Abu Ghraib," Lieutenant Colonel Guy Rudisill, spokesman for detainee affairs, told Reuters.

"They detonated two VBIEDs (suicide car bombs) and also fired rocket-propelled grenades into the prison camp ... it was a sustained attack," he said. Mortars and small arms fire were also directed toward the prison, on Baghdad's western edge.

"The attacks were intermittent. They would fire RPGs and then stop, then they would attack again," Rudisill said.

U.S. forces responded with heavy weapons, eventually bringing the situation under control. It was not known how many insurgents were wounded or killed in the battle.