The U.S. Defense Department says violence in Iraq has fallen to a four-year low, with some types of attacks down 80 percent from last year.

But, the Pentagon's quarterly report issued Monday also warns that Iraq's security gains are "fragile, reversible and uneven."

Admiral Mike Mullen (file photo)

U.S. military chief Admiral Mike Mullen says Iraq has become a "better place" politically, economically and from a security standpoint since last year. But, he says Iraq has not yet reached a point where that progress is sustainable and irreversible.

In the latest violence Monday, an Iraqi gunman killed two American soldiers and wounded three other U.S. troops and their interpreter south of Baghdad.

Iraqi police say a municipal official opened fire on the U.S. troops as they left a meeting with local authorities in the town of Madain. The soldiers fired back and killed the gunman, whose motive was not clear.

In another development, Iraq Prime Minister Nouri al-Maliki says his counter-insurgency campaign will move next to Diyala province, north of Baghdad.

Mr. Maliki was speaking Monday on a visit to the southern province of Maysan, where Iraqi forces launched a crackdown on Shi'ite insurgents last week. He promised to keep Iraqi forces in Maysan until, in his words, criminals and killers have no chance of returning. Iraqi forces have met no resistance in Maysan as they arrested suspected militants and confiscated weapons.

A second U.S. report issued Monday by the congressional Government Accountability Office also notes an overall reduction in violence in Iraq.

The report says Baghdad has enacted key legislation to give amnesty to detained Iraqis and to allow some members of the former ruling Ba'ath party to return to government. But, it also says Iraq's progress is fragile and many unmet goals and challenges remain.

The GAO report says only about 10 percent of Iraqi military units are capable of operating without U.S. assistance.

Some information for this report was provided by AFP and AP.