By Dean Yates BAGHDAD, Feb 7 (Reuters) - The U.S. military said a transport helicopter had come down near Baghdad on Wednesday, the fifth helicopter to be lost in Iraq in three weeks. Iraqi witnesses reported seeing an aircraft in trouble during gunfire from the ground. U.S. military spokesman Major-General William Caldwell told reporters it would be inappropriate to talk about casualties. "We have a CH-46 that is down," Caldwell said, referring to the twin-rotor Sea Knight, the Marine version of the Chinook, which can carry up to 25 passengers and four crew. "We have a quick reaction force on site." U.S. and Iraqi forces are preparing to launch a major offensive against militants in Baghdad that is regarded as a last-ditch attempt to prevent all-out civil war. The U.S. military said "some new elements" of the plan had begun to be implemented although Iraqi officials said preparations were still being carried out. "It is not going to be a sudden effort, it will be a gradual effort ... People have to be patient," Caldwell said. An Iraqi policeman who had been to the area northwest of Baghdad where the helicopter came down told Reuters he had seen wreckage. Another witness also said he had seen wreckage. The U.S. military said on Sunday it was adjusting its tactics after four helicopters were shot down in separate incidents over a two-week period. Twenty one U.S. servicemen and private security contractors were killed in those attacks. The high number lost in such a short time has raised questions about whether militants have changed tactics or are using more sophisticated weapons. BAGHDAD OFFENSIVE Iraqi government spokesman Ali al-Dabbagh said preparations for the Baghdad offensive were still being worked out. "There will be no sharp start for this operation," he said. Caldwell said the operation would take time to build up. He said an Iraqi general, Abboud Qanbar, would head the campaign. U.S. forces will not take orders from the Iraqis. Iraqi Prime Minister Nuri al-Maliki ordered his military commanders on Tuesday to speed up preparations for the crackdown after a string of attacks killed hundreds of people in recent days. There is growing frustration among Iraqis over the delay in launching the offensive that Maliki first announced nearly a month ago. Iraqi officials say the planned crackdown was due to have started this week but that Iraqi security forces had asked for more time to get their troops in place. Critics say a previous offensive last summer failed partly because too few Iraqi troops were involved. U.S. President George W. Bush has committed 17,500 more troops to the Baghdad push. Most of the extra troops promised by Bush have yet to arrive. (Additional reporting by Ibon Villelabeitia, Mariam Karouny, Aseel Kami and Ross Colvin)
