Gen Petraeus took over as head of US forces in Iraq in January

Petraeus interview

Speaking to the BBC's John Simpson in Baquba, Gen Petraeus said there was evidence that the recent troops surge was producing gains on the ground.

But he warned that US forces were engaged in a "tough fight" which will get "harder before it gets easier".

His comments come as US calls for a rapid troop withdrawal gather strength.

Gen Petraeus was keen to emphasise that the ongoing unrest in Iraq is not something he expects to be resolved overnight.

"Northern Ireland, I think, taught you that very well. My counterparts in your [British] forces really understand this kind of operation... It took a long time, decades," he said.

The question is how can we gradually reduce our forces so we reduce the strain on the army, on the nation and so forth

Gen Petraeus

Battle looms in US Congress

"I don't know whether this will be decades, but the average counter insurgency is somewhere around a nine or a 10 year endeavour."

He went on to say that more important than the length of time it would take to stabilise Iraq was the number of US troops which would be required to remain in the country.

"I think the question is at what level... and really, the question is how can we gradually reduce our forces so we reduce the strain on the army, on the nation and so forth," he said.

He said everyone wanted the US forces to be able to leave, both Americans and Iraqis alike, but he said it was vital to ensure that "the gains that have been hard fought in places like Baquba and Ramadi could be sustained, maintained and even built on by Iraqi forces and Iraqi political leaders".

The violence has subsided in Ramadi over the past six months - largely, correspondents say, because tribes have turned against al-Qaeda. The Americans have taken parts of Baquba, but it is still unclear how much they control.

'Surge taking hold'

Gen Petraeus attributes this success to the recent surge in combat troop numbers, under which some 30,000 extra US troops have been deployed in Iraq, saying that although much work still remains to be done "the surge is achieving progress on the ground".

US forces recently made gains in the insurgent stronghold Baquba

"In Baghdad for example, June was the lowest month for sectarian deaths in a year."

By contrast, April, May and June were the deadliest three months for US troops since the invasion of Iraq in March 2003.

About 3,600 US soldiers have been killed in Iraq since the conflict began.

Gen Petraeus insisted it was not yet possible to see the full effect the surge was having as it took until mid-June to get all of the additional troops and equipment on the ground.

"I think again we need to see where we are in September when we'll have had a couple of months of all of our forces. We are still in the first month of the surge of operations that is following now the surge of forces," he said.

Time running out

Gen Petraeus is due to return to Washington in September to report on the campaign's progress.

In recent days four Republican senators have withdrawn support for President George W Bush's Iraq strategy, adding their voice to a growing number calling for a new plan.

And this week will see a contentious debate in the US Senate over a major defence spending bill.

On Sunday the Pentagon announced that US Defence Secretary Robert Gates was cancelling a planned Latin American tour in order to focus on the upcoming clash.

According to BBC world affairs editor John Simpson, the debate is moving so fast in Washington that Gen Petraeus's efforts, which might have saved the day for the Bush administration if they had been introduced three, or even two, years ago, may well have come too late.