BAGHDAD, Iraq (CNN) -- The top-ranking U.S. military official in Iraq estimated Friday that it may be late summer before it's known if the latest strategy to secure Baghdad is working and residents "feel safe in their neighborhoods."
Gen. George Casey also gave that time frame as the projected length of time the new troop deployment in Iraq will last.
U.S. and Iraqi officials are working on a new effort to restore law and order in Baghdad and other parts of Iraq. President Bush announced a plan last week to send 21,500 additional troops to Iraq, primarily in the capital.
Casey made his remarks while accompanying U.S. Defense Secretary Robert Gates on an unannounced stop in Iraq. The trip to Iraq is Gates' second since taking his post last month.
The defense chief traveled to the southern city of Basra, where British forces are based, and then to Tallil Air Base in Nasiriya, also in southern Iraq, to meet with U.S., British and coalition officials.
"Given what is stake, failure is not an option," Gates said, in referring to the U.S. operation in Iraq.
"There is universal agreement, on the importance of success here in Iraq, and on confronting extremism here."
Gates and Secretary of State Condoleezza Rice, who also traveled to the Mideast and Europe this week to sell Bush's plan, are scheduled to meet Saturday with the president at Camp David, Maryland, according to a White House official.
Baghdad violence caps bloody weekGates' visit to Iraq coincided with attacks around Baghdad that left at least four people dead and the discovery by police of 18 bodies there, an Iraqi Interior Ministry official said.
Among the bodies found was a tribal leader from Falluja who was shot dead in western Baghdad. The other bodies went unidentified, and most of them showed signs of torture, the official said.
Mortar rounds killed a woman and wounded three civilians in southern Baghdad's Dora neighborhood, and additional mortars killed two civilians and wounded three, also in southern Baghdad. A fourth civilian died and three were wounded when gunmen began firing randomly, the Interior Ministry official said.
The violence capped a bloody week, underscored by an attack Tuesday at Mustansiriya University, where 70 people were killed.
Al-Sadr aide arrestedBacked by U.S. troops, Iraqi forces captured a top aide to radical Shiite cleric Muqtada al-Sadr in an overnight raid in Baghdad, a spokesman for the cleric's militia said Friday.
Sheikh Abdul al-Hadi Darraji -- director of al-Sadr's main offices in the Shiite slum of Sadr City -- was arrested at a mosque in the eastern Baghdad neighborhood of Belediyat, the spokesman said.
Al-Sadr's Mehdi Army militia is part of a politically powerful Shiite movement thought to be in the middle of the Sunni-Shiite sectarian violence in Iraq.
A U.S. military statement Friday did not name Darraji but did announce U.S. and Iraqi forces had arrested a "high-level, illegal armed group leader" accused of kidnapping, torturing and killing Iraqi civilians while heading an "illegal armed group punishment committee."
In addition, the "armed group leader" is suspected of working with "death squad commanders" and armed cells that practice sectarian revenge killings in Baghdad.
Another al-Sadr official, Mohammed al-Kaabi, said members of the cleric's movement were in talks with Iraqi Prime Minister Nuri al-Maliki and other officials to secure Darraji's release.
"We are very surprised over this arrest; we think there was a mix-up," al-Kaabi said, adding that Darraji has "always called for calm and rising above sectarian division."
He said Darraji was a public relations spokesman not involved with politics or military affairs.
The U.S. military also announced the arrest of two other suspects in the operation.
Al-Kaabi disputed that account, saying troops detained six people and later released two, leaving Darraji and three others in custody.
Earlier this week, the prime minister said that security forces recently had cracked down on the Mehdi Army, including the arrests of 400. (Full story)
Al-Maliki has been seen as reluctant to take on the Mehdi Army because support from al-Sadr helped him become prime minister.
In an interview published Friday with an Italian newspaper, al-Sadr said his militias would not retaliate during the Muslim holy month of Muharram, which begins Friday for Sunnis and Saturday for Shiites, The Associated Press reported.
He also told La Repubblica he was a target, saying, "I have moved my family to a secure location. I even have had a will drawn up, and I move continuously in a way that only few can know where I am," according to the AP.
Other developmentsA U.S. soldier was killed when a roadside bomb exploded Thursday during a patrol in Baghdad, the U.S. military said Friday. Three other soldiers were wounded in the blast. The number of U.S. troops killed in the Iraq war stands at 3,021. Seven American civilian contractors of the military also have died.
A Marine corporal pleaded guilty Thursday to kidnapping and murdering an unarmed Iraqi civilian last year, saying he and other servicemen pursued the man because they were "sick and tired of getting bombed," the AP reported. In all, seven Marines and a Navy medic have been accused in the killing of 52-year-old Hashim Ibrahim Awad in Hamdania.(Full story)
A proposed law that would reverse decades of state controlled-oil and formalize the channeling of revenues to the Kurdish north and Shiite south is set to be forwarded next week for a vote in Iraq's parliament, a government spokesman said Thursday. The stakes are enormous, as estimates of Iraq's oil reserves range from 120 billion to 250 billion barrels, making Iraq the world's third or second largest oil producer.
CNN's Yousif Bassil, Sam Dagher, Arwa Damon, Susan Garraty and Jomana Karadsheh contributed to this report.