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

The 3.5-square-mile district along the Tigris River in the center of Baghdad includes the U.S. and British embassies as well as Iraq's parliament and the offices of Prime Minister Nouri al-Maliki.

Attacks against the Green Zone have increased in recent months, adding to the concern over the safety of key Iraqi and international officials who live and work in the zone.

In a report last month, the United Nations office in Baghdad said the "threat of indirect fire" ¡ª meaning rockets and mortars ¡ª into the Green Zone had increased, adding that the barrages had become "increasingly concentrated and accurate."

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, the report said.

No group claimed responsibility for the barrage, which began after 4 p.m. 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.

Security officials had warned of a heightened security threat against the Green Zone following a series of U.S. military operations 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.

Attacks against the Green Zone have continued despite the Baghdad security crackdown, which began in mid-February. U.S. Ambassador Ryan Crocker and the top military commander Gen. David Petraeus are due to give a preliminary report to Congress this week outlining progress in efforts to improve security here.

The report is due as opposition to the U.S. mission is increasing in Congress, even among President Bush's own party.

Attacks against the zone have also renewed concern about security at the new U.S. Embassy, which is due to open this year within the protected area. The embassy will be the world's largest and most expensive foreign mission, though it may not be large enough or secure enough to cope with the chaos in Iraq.

Also Tuesday, at least 46 violent deaths were reported around the country. In Baghdad, police reported finding the bodies of 23 people tortured and slain, presumably by Shiite and Sunni death squads which still operate despite the security crackdown. Sixteen of their bodies were found on the west side of the city, which is more religiously mixed.

The British military said Tuesday warplanes struck the day before in the southern town of al-Majar al-Kabir near the Iranian border, killing three militants suspected of smuggling weapons into Iraq.

Iraqi police officials said a British helicopter strike killed the brother and two guards of radical Shiite cleric Sheik Abu Jamal al-Fartousi, whom the British military accused of being a leader in Iran's elite Quds Force suspected of arming militants.

The U.S. military said American special operations forces in a raid Sunday captured 12 militants in Baghdad who had broken away from the Mahdi Army and had carried out attacks on U.S. and Iraqi troops