Washington  - General David Petraeus, the top US commander in Iraq, said Tuesday the increase of US soldiers has dramatically reduced violence but warned the progress is "fragile" and "reversible."

"Though Iraq remains a violent country we do see progress in the security arena," Petraeus said at the opening of two days of congressional testimony to provide an update on the situation in Iraq along with US ambassador Ryan Crocker.

Petraeus was expected to tell the Senate Armed Services Committee that there should be no additional withdrawals of US troops when the troop surge ordered by President George W Bush last year ends in July.