A woman suicide bomber struck Shi'ite pilgrims south of Baghdad on Thursday, killing at least 18 people and wounding scores of others after the government announced new measures to protect worshippers ahead of a major religious festival. Also Thursday, the US military announced that six Navy guards face trial for allegedly assaulting prisoners and releasing pepper spray into a cellblock following a disturbance at the main US prison in Iraq. The woman bomber detonated her explosives among a group of pilgrims resting by the side of a road in Iskandariyah, a former Sunni insurgent stronghold 30 miles south of Baghdad. Women were cooking dinner, men were praying and children were playing nearby when the attacker struck, a witness said. "Minutes after I passed the resting spot, I heard a big explosion. I turned my head and saw big flames," said Ahmed al-Saadi, a 34-year-old carpenter from Baghdad's Sadr City district. "We rushed to the scene and saw charred bodies, while wounded people were crying for help. Pots and burned prayer rugs were scattered all over the place." There were conflicting casualty tolls as is common in such attacks where bodies are mangled and identification is difficult. The US military put the death toll at 18, including one policeman and 17 civilians, and said a lone woman bomber was responsible. A senior provincial security officer said 26 people were killed and 75 wounded. He spoke on condition of anonymity because he was not supposed to release the figure. The fact that such a brazen attack could take place in an area where US and Iraqi officials had touted major security improvements is an ominous sign of the risks still posed by extremists. The pilgrims were marching to the Shi'ite holy city of Karbala, 50 miles south of Baghdad, to celebrate the birth of Mohammed al-Mahdi, the 12th Shi'ite imam, who disappeared in the ninth century. Devout Shi'ites believe he will return to restore peace and harmony. The ceremonies reach their high point this weekend. Witnesses interviewed in an Iskandariyah hospital spoke of horrific scenes of mangled flesh and screaming victims. Khazim Jabir, 39, from the Baghdad neighborhood of Hurriyah, said he was walking near a group of pilgrims receiving food and water when he heard a thunderous explosion. "Seconds later, I saw fire and flaying pieces of flesh," he said. "I fell down and I was able to see one of my legs bleeding. People were running in all directions." Yassir Ibrahim, 28, also from Baghdad, said his hands were injured by the blast. "I don't understand the reason for attacking us," he said. "We were only heading to a holy place." Earlier Thursday, at least two people were killed and 16 wounded in a pair of small bombings in Baghdad that police believed were targeting pilgrims. Every year since the 2003 collapse of Saddam Hussein's Sunni-dominated regime, hundreds of thousands of Shi'ites have celebrated religious festivals - and Sunni extremists have often targeted them. Last month, three women suicide bombers attacked Shi'ite pilgrims during a festival in Baghdad, killing at least 32 people and wounding more than 100. No group claimed responsibility for the Thursday blast, but attacks on Shi'ites have been associated with the Sunni extremist al-Qaida in Iraq. However, US spokesman Lt. Col. Steven Stover said at least one of the Baghdad bombings was believed carried out by Iranian-backed Shi'ite extremists "in an attempt to heighten public unrest." With security concerns on the rise, the Iraqi military announced a series of measures aimed at protecting pilgrims and tamping down sectarian tensions during the festival period. Iraqi military spokesman Brig. Gen. Qassim al-Moussawi said members of the Shi'ite-dominated security force were banned from placing religious posters on their vehicles or joining pilgrims in chanting Shi'ite slogans - moves that might provoke Sunnis. Al-Moussawi said pilgrims must not carry weapons and travel by foot after sundown. He also warned pilgrims to avoid rumors that could spread panic or accept food from strangers. In Karbala, police deployed about 2,000 additional troops in the city. Bikes and motorcycles were also banned from the center of the city around the two major religious shrines. The charges against the six Navy guards were in connection with a May 14 incident at Camp Bucca, the biggest American detention facility near the Kuwaiti border, according to Navy spokeswoman Cmdr. Jane Campbell. Trouble started when several guards were attacked by detainees, who spit and threw containers of human waste at them, Campbell said. Two detainees were allegedly beaten and eight others were locked overnight in a cell which was filled with pepper spray and the ventilation was shut off, she said. Seven other sailors received nonjudicial punishment for failing to report the incidents, Campbell said. Two had their charges dismissed while others were reduced in rank or faced suspended punishment, she added, declining to be more specific. Trials for the six others are expected to be held within 30 days, Campbell said. Camp Bucca houses about 18,000 of the 21,000 detainees held by the US in Iraq. Treatment of detainees drew international criticism in 2004 with the release of pictures of grinning US soldiers posing with detainees at the Abu Ghraib prison west of Baghdad. Some were naked, being held on leashes or in painful and sexually humiliating positions. That prison has since been closed, and 11 US soldiers were convicted of breaking military laws. Five others were disciplined in the scandal. The US has announced a series of reforms of the detention system, including Muslim religious training by moderate imams of younger inmates deemed less extreme in their views. Last month, the military confirmed that Iraqi detainees had maintained self-styled Islamic courts and tortured or killed inmates who refused to join them. US officials said the problem was centered at Camp Bucca.