BBC News" said:Blair defiant over Iraq security
Some 7,000 UK troops are currently serving in Iraq
Tony Blair has refused to apologise for the security situation in Iraq, saying he bears no responsibility for it.
The prime minister told BBC Radio 4 extremists trying to thwart Iraq's democracy were to blame for attacks.
"I don't think we should be apologising at all for what we are doing in Iraq. We're trying to support the democrats against the terrorists," he added.
The UK is to withdraw 1,600 troops from Iraq but Mr Blair said numbers could increase again "if we're needed".
He told MPs on Wednesday that the remaining 5,500 troops would stay until 2008.
However, when he was asked about reversing that decision on the Today programme, he said: "I don't want to get into speculating about that because we have the full combat capability that's there.
No Iran strike plans, says Blair
"So, if we're needed to go back in any special set of circumstances we can, but that's not the same as then increasing back the number."
He said he bore no responsibility nor would he apologise for the "terrible" security situation in Iraq.
"I don't think we should be apologising because we're not causing the terrorism.
"It's being caused by internal extremists who are linking up with external extremists. And 80-90% of that violence is in or around Baghdad and it's being caused deliberately in order to stop the government of Iraq - which 12 million people voted for - from functioning."
He added: "Although the situation is terrible...we should be immensely proud of the work we are doing to try to help Iraq get on its feet and be the country its people want it to be."
His assessment was at odds with comments by the former British envoy to Iraq, Sir Jeremy Greenstock.
Sir Jeremy, in a BBC documentary, said Mr Blair had taken his "eye off the ball" in the aftermath of the war and that no one in those early days had focussed on the security of Iraq.
"There was a vacuum from the beginning into which looters, saboteurs, the criminals, the insurgents moved very quickly."
Mr Blair dismissed suggestions coalition forces had been unprepared for the aftermath of war, particularly the sectarian violence.
"There was no way that the Iraqi police force that was there under Saddam was going to be able to keep order in the country properly.
"They were an instrument of Saddam's dictatorship and therefore you were always going to have to build the Iraq police and army from scratch.
"That was what we were going to have to do.
"Now, you can argue about - and I've already been back over it myself - about whether you could have disbanded the army differently and so forth, but it isn't the principal reason you've got a problem in Iraq.
"The principal reason you've got a problem is, as I say, because there are elements deliberately giving you that problem."
During the BBC interview, Mr Blair also said he knew of no plans to attack Iran. There has been tension between the US and Iran over allegations Tehran was building nuclear weapons as well as aiding Iraqi insurgents.
Mr Blair said "people are pursuing a diplomatic and political solution".