BAGHDAD -- U.S. officials said four Americans have been killed in an explosion at a local council building in Baghdad's Shiite Sadr City district.

Iraqi police officials said at least six Iraqi civilians also were killed and 10 others wounded in Tuesday's blast, which apparently was targeting Americans visiting the building.

The U.S. military and the embassy said two American soldiers and two American government employees were among the dead.

An embassy spokeswoman said the government employees were civilians with the State Department and the Defense Department.

The explosion came a day after a gunman killed two U.S. soldiers attending a municipal council meeting southeast of Baghdad. Four other Americans were wounded in that attack.

Their increased presence in local communities has made them more vulnerable to attacks, but American commanders say it's necessary.

U.S. military officers have been working vigorously to restore and promote local administrations amid a sharp drop in attacks over the past year, with the goal of preventing areas from falling back under the control of rival Sunni and Shiite extremists.

On Monday, the Pentagon released two reports that suggest Iraq is is much less violent now than it was a year ago, but it still not ready to stand alone.

The number of weekly attacks in Iraq has dropped from about 1,200 a week in June 2007 to about 200 a week now, a U.S. commander said Monday. The military also recorded a 70 percent decrease in roadside-bomb attacks and an 85 percent spike in the number of weapons caches coalition forces have found over the past year.

But the quarterly Iraq progress report warned that Iran and Syria continue to provide safe havens for terrorists, allowing them to cross the borders into Iraq. It also repeated concerns that Iran continues to supply both weapons and training for militants in Iraq.

On the domestic side, the report said the Baghdad government "lacks the ability to execute programs on the scale required." It added that economic improvements remain "fragile, reversible and uneven."

Joint Chiefs of Staff Chairman Adm. Mike Mullen offered hope that troop levels in Iraq will continue to come down this year.

Mullen said continued improvement in Iraq will allow him to free some forces by the fall to send to Afghanistan.