Overall levels of violence in Iraq have fallen 80 percent, US Defense Secretary Robert Gates said on Tuesday, but he cautioned against using the improved security to speed up the withdrawal of US troops.

US Democratic presidential candidate Barack Obama has called for US troops to be withdrawn within 16 months while his Republican rival, John McCain, has warned that pulling out the troops too quickly could undo the progress made in stabilizing Iraq.

"I worry that the great progress that our troops and the Iraqis have made have the potential to override a measure of caution born of uncertainty," Gates told a congressional hearing in Washington.

"The continuing but carefully modulated reductions the president has ordered represent the right direction and the right of course of action," he said.

US President George W Bush announced earlier this month that 8,000 troops would be withdrawn by February without being replaced. Gates said the withdrawal was already under way. - Reuters