There has been a significant decline in attacks carried out by Iran-backed Shiite militias in Iraq in recent weeks, after June saw the highest US death toll in two years, the top US officer said on Tuesday.

Admiral Mike Mullen, the outgoing chairman of the Joint Chiefs of Staff, made the remarks during a visit where he pressed Iraqi leaders to decide on whether they wanted an extended American troop presence beyond a year-end withdrawal deadline.

"We have seen a dramatic reduction in these deadly attacks," Mullen told reporters at a news conference at the US military's Victory Base Camp on Baghdad's outskirts.

He did not give specific figures.

Mullen said US forces had voiced concern over attacks by Shiite Muslim extremist groups in talks with Iraqi leaders, and added "they've taken some significant steps internal to Iraq and... they've addressed it in Tehran."

"We've seen a pretty dramatic drop from where we were in June in the last two to three weeks. One of the points I emphasise is that drop needs to be sustained."

Overall, 14 American soldiers were killed in Iraq in June, the worst month for US forces here since 2009.

Five died in July, four in "hostile" incidents, bringing the overall number of US troop fatalities in Iraq since the 2003 invasion that ousted Saddam Hussein to 4,474, according to data compiled by independent website

The US Special Inspector General for Iraq Reconstruction, Stuart Bowen, warned in a report published on Saturday that the country was less safe than one year ago and that security was deteriorating.