(Adds Gates quotes, paragraphs 5-6, 13 and 15)
By Peter Graff
BAGHDAD, Jan 10 (Reuters) - U.S. warplanes launched their biggest air strike in Iraq since at least 2006 on Thursday, bombarding date palm groves on Baghdad’s southern outskirts with more than 40,000 pounds of bombs in a matter of minutes.
Two B-1 bombers and four F-16 fighter jets struck more than 40 al Qaeda targets in three zones of Arab Jabour, a lush district just south of the capital that has become a haven for fighters driven out of other areas.
"Thirty-eight bombs were dropped within the first 10 minutes, with a total tonnage of 40,000 pounds," the military said in a statement. "Each bomber passed over twice and the F-16s followed to complete the set."
The attack formed part of Operation Phantom Phoenix, a major countrywide offensive against al Qaeda guerrillas that U.S. forces announced this week.
U.S. Defense Secretary Robert Gates said if U.S. forces were able to oust the militants from their current enclaves, there were not many other places they could go. But he warned that U.S. troops still faced a difficult mission.
"There will continue to be tough days and tough weeks. We are not done yet, by any means," Gates said at a Pentagon news conference with Iraqi Defense Minister Abdel Qader Jassim.
U.S. forces spokesman Maj. Winfield Danielson said Thursday’s air strike was the biggest in Iraq since at least 2006. A spokeswoman for U.S. forces in central Iraq, Maj. Allayne Conway, said it was too soon to assess the damage inflicted.
"We certainly have our opponents on the ropes and we’re going to go after him while he is on the ropes," said Lt.-Col. Robert Wilson, deputy commander of the 3rd Combat Aviation Brigade, 3rd Infantry Division, in a statement.
Large-scale air strikes have been rare in Iraq, especially over the past few months when the intensity of military action tapered off as overall violence declined and U.S. commanders emphasized "hearts and minds" engagement with civilians.
In televised remarks to security officials, Iraq’s Prime Minister Nuri al-Maliki said: "The sectarian violence has ended and we are now aiming to complete the national reconciliation process with the whole spectrum of the Iraqi people."
But the operation launched this week shows a renewed determination by U.S. forces to use traditional combat power against a stubborn al Qaeda enemy that has not lost its ability to launch attacks despite being driven from most areas.
TOLL ON US SOLDIERS
The offensive has taken its toll on American forces as well. After a month in which the rate of U.S.-led coalition deaths fell to fewer than one per day for the first time since 2004, nine American soldiers were killed in 48 hours.
Gates said the deaths were a "stark reminder of the work that remains to be done."
Six American soldiers were killed on Wednesday by an explosion in a booby-trapped house in Diyala province, and three others were killed on Tuesday in Salahuddin province, two northern areas where U.S. forces say al Qaeda has regrouped.
"Because we are on the offensive again in areas where we have not been active for some time, it’s not a surprise that we will see some higher casualties until that area is cleared," Gates said.
Operation Phantom Phoenix has so far included a large-scale sweep in Diyala by thousands of U.S. and Iraqi troops, and smaller operations across the North and Baghdad’s outskirts.
The U.S. military says al Qaeda Sunni Arab militants have been driven out of most of the territory they once held in Iraq, especially the West of the country and parts of Baghdad, and overall violence declined dramatically in the second half of 2007.
But militants have regrouped in three provinces north of Baghdad and in palm groves on the capital’s southern outskirts.
They have stepped up so-called "spectacular" attacks — suicide bombings which often kill large numbers of people — launching major strikes nearly every day of the past two weeks mainly against neighborhood patrols paid by U.S. forces.
The war has forced more than 3 million people to leave their homes. Some have started to return, but the International Organisation for Migration said in a report that those who have gone home so far represent only a "minute percentage."
"Despite decreased violence, slowing displacement rates and limited returns in 2007, population displacement within and from Iraq remains one of the largest and most serious humanitarian crises in the world," the IOM report said.
The United Nations’ World Health Organisation released figures on Wednesday estimating about 151,000 Iraqi civilians had died violently in Iraq in the war’s first three years, with the exact figure falling between 104,000 and 223,000.
The WHO figure, based on a survey of 10,000 Iraqi households, does not include deaths after June 2006. The 12 months that followed were the deadliest year of the war. (Additional reporting by Andrew Gray in Washington; Editing by Cynthia Osterman)