Iraqi forces could be up to strength in about a year but it would be foolhardy to set a date for UK troops to pull out, the head of the Army has said.
97 UK soldiers have died in Iraq since military action began
General Sir Mike Jackson said progress was being made on security in Iraq.
But the decision to withdraw would be complicated and the timing could vary from region to region, he said.
The chief of the general staff also said some insurgents had infiltrated Iraqi security forces - something he branded "reprehensible".
'Sense of proportion'
The UK has said it will withdraw troops for Iraq once Iraqi forces are able to take charge of security.
Sir Mike told BBC Radio 4's Today programme there was a varied picture on security in Iraq but it seemed the political process was moving on and the growth of the Iraqi security forces was impressive.
"It is true that the security level, and the incidents are anything than what we would want at the moment and only yesterday very sadly we lost another British soldier," he said.
Sir Mike called for a sense of proportion and said such attacks were not supported by the vast majority of Iraqis.
But he admitted Iraqi forces had been infiltrated to some degree by "people whose loyalty is to some sectoral group, and not to the people as a whole".
"This is obviously not the right answer, and we are taking steps alongside the Iraqis themselves, to put this right.
"We cannot accept that policemen, whose job it is to serve the community as a whole, are behaving in any form of criminal way. That is simply not on. It is not on in any country.
"Where there is clear evidence that policemen have been committing criminal acts, they will be arrested, and the process of law will be applied against them."
Sir Mike said there was also a problem of "elements" from Iran interfering in Iraq with attacks on British troops - although he did not necessarily blame the Iranian Government itself.
Pressed on the timetable for withdrawing British troops, Sir Mike said it would be "foolhardy in the extreme" to set a date for pulling out.
"The plans for building the Iraqi security forces are very clear," he said. "In terms of numbers and what must be done, we are planning over the next 12 months or so. It's that sort of framework
"But that does not of itself bring you to the conclusion that at that point, that is the moment [for withdrawing UK troops].
"It is a question of progress - where this insurgency is- and an agreement with the Iraqi government."
The Chief of the Defence Staff, General Sir Michael Walker, has suggested Iraq has damaged Army recruitment.
"We are, if you like, guilty by association with a decision to go to war that not the whole of this country enjoined," he has said.
As head of the Army, Sir Mike Jackson said recruiting targets were missed in the last financial year by 7%.
Iraq was "bound to be a factor" when young people and their families considered joining up, he said.
But this year's recruiting targets were so far on target to be hit, he said.