(In the penultimate paragraph, please read "Until the job is done" instead of "Until the job is down".) BAGHDAD, Jan 15 (Reuters) - The first additional American troops who will take part in a major new security plan in Baghdad have arrived in the Iraqi capital, the U.S. commander in Iraq, General George Casey, said on Monday. "The initial elements of the first group are here," he told a news conference. He declined to give more details or say when operations would start but he said a report that 4,000 of the planned 17,500 extra troops had arrived was "real high". Asked about reports of frustration among U.S. officials with Iraqi counterparts over the planning of an operation to be led by the Iraqi government with U.S. help, Casey said: "Transitions generate friction. And we are in a period of transition." But he said U.S. and Iraqi officials were working hard to ensure a workable command structure that would allow Iraqi control while maintaining the security and effectiveness of U.S. troops. "We are working out understandings at every level to ensure there are no misunderstandings," Casey said. "This is a plan that is Iraqi-conceived but we have been involved every step of the way," he said. "American forces will remain under American command ... Our forces will not be put at risk because of the command relationship." He was confident the Iraqi government would "purge" elements from its security forces which were loyal to militias and other groups outside the government. Casey cautioned the Iraqi public not to expect an instant improvement in security in the capital. "It's not going to happen overnight," he said, speaking of a "gradual evolution" over two to three months, with the hope of showing results in the summer or autumn. Asked how long it would last, he said: "Until we succeed ... until the job is done." Senior Iraqi officials told Reuters it would last at least six or seven months.
