Iraqi boys peer into the shell of a car that was destroyed in an overnight raid by U.S. and Iraqi troops in the Shiite enclave of Sadr City of Baghdad. Police said two were killed and four injured by U.S. and Iraqi forces conducting house-to-house searches Tuesday in three sections of the neighborhood. Two cars were burned along with a store in the raids, which started at 2 a.m. At least 46 violent deaths were reported Tuesday across the country.

Baghdad ? Extremists unleashed a barrage of more than a dozen mortars or rockets Tuesday into the Green Zone, killing at least three people – including an American – and wounding 18 in an area once considered the safest in the Iraqi capital.

An Iraqi and a “third country national” also were killed in the attack, the U.S. Embassy said in a statement. The embassy said the 18 wounded included five Americans – two military members and three civilian contract employees.

The attack against the well-protected nerve center of the U.S. and Iraqi leadership occurred at a time of mounting opposition to the war in Congress and is likely to cast doubts on claims that the U.S. troop buildup is bringing stability to the Iraqi capital.

Attacks against the Green Zone have increased in recent months, adding to concern about the safety of key Iraqi and international officials who live and work in the 3.5-square-mile district along the Tigris River in the center of Baghdad. The zone includes the U.S. and British embassies as well as Iraq’s parliament and the offices of Prime Minister Nouri al-Maliki.

No group claimed responsibility for Tuesday’s barrage, which began after 4 p.m. and sent clouds of white smoke billowing up from inside the Green Zone with each detonation. But some of the fire appeared to have come from the eastern side of the city where the Mahdi Army militia of Shiite cleric Muqtada al-Sadr operates.

At least three loud explosions were heard after midnight from Shiite militia strongholds in eastern Baghdad, but it could not be determined whether military operations were under way there.

Security officials had warned of a heightened security threat against the Green Zone following a series of U.S. military strikes against Shiite militias suspected of ties to Iran. In the latest attack, a joint U.S.-Iraqi force raided parts of Sadr City before dawn Tuesday.

In a report last month, the United Nations office in Baghdad said the “threat of indirect fire” – meaning rockets and mortars – into the Green Zone had increased.

The report said such attacks increased from 17 in March to 30 in April and 39 by May 22. Between Feb. 19 and the end of May, at least 26 people were killed in Green Zone attacks.