Attacks on US and Iraqi troops and civilians reached their highest level since Iraqi sovereignty was restored in June 2004, a Pentagon report has said.

It says the number of attacks rose to almost 1,000 a week in recent months.

The worst violence was in Baghdad and the western province of Anbar, long the focus of activity by Sunni insurgents.

The report comes hours after new US Defense Secretary Robert Gates said failure in Iraq would be a "calamity" that would haunt the US for many years.

For the Bush administration, this latest evidence will add to the sense of urgency in Washington to find a fresh strategy in Iraq, the BBC's Nick Miles in Washington says.

Escalating violence

The quarterly report - entitled Measuring Stability and Security in Iraq - said there was an average of 959 attacks a week between 12 August and 10 November.

This constituted a 22% increase in attacks and a 2% jump in civilian casualties, compared to the three previous months.

The report said that 54% of all attacks took place in Baghdad and Anbar.

Most casualties were Iraqi, despite the fact that 68% of the attacks targeted US-led coalition troops.

The Pentagon document said radical Shia cleric Moqtada al-Sadr's Mahdi Army militia has replaced al-Qaeda in Iraq as the single largest threat to the country's security.

It also criticized the Iraqi government's efforts to end sectarian violence.

"The violence in Iraq poses a grave threat to political progress [in Iraq]," the report said.

The latest findings seem to chime with President Bush's emphasis on shifting responsibility for security to the authorities in Baghdad, our correspondent says.

Gates warning

Mr Bush is under increasing pressure to consider a new US strategy for Iraq.

He is expected to indicate within the next few weeks whether that will mean more US troops in Iraq or a possible phased withdrawal.

The US currently has about 140,000 troops in Iraq.

The report was released shortly after Mr Gates took his oath of office as new US defense secretary, replacing Donald Rumsfeld.

Mr Gates said Iraq was his top priority, adding that he would soon visit the country to hear the "unvarnished" views of US commanders on how to improve matters.

"Failure in Iraq at this juncture would be a calamity that would haunt our nation, impair our credibility, and endanger Americans for decades to come," he said.

Mr Gates also vowed not to let Afghanistan become "a sanctuary for extremists" again.

Photo Caption

US soldiers at the site of a car blast in Kirkuk /file photo