The number of United States military personnel killed in Iraq since the US-led invasion five years ago has passed the 4,000 mark

The latest to die were four soldiers whose patrol vehicle was blown up by a bomb in southern Baghdad on Sunday

The deaths bring the total to 4,000, according to the US military and independent monitoring groups

In other violence, Baghdad's Green Zone came under fire, and a suicide bomber killed 13 Iraqi soldiers in Mosul

In Baghdad, the heavily-fortified Green Zone suffered sustained mortar and rocket fire, which killed at least 15 civilians.

Sunday's violence underlines the fragile, reversible nature of the apparent improvements in security, say correspondents.

There have been about 15 soldiers wounded for every fatality in Iraq

"We are always quick to note that the progress is tenuous and that it is reversible," said the top U.S. commander in Iraq, Gen. David Petraeus, "and that there are innumerable challenges out there."