Baghdad, Iraq ? Mortar rounds hammered the U.S.-controlled Green Zone for a second day Wednesday, killing at least two Iraqis, wounding about 10 more and raising new fears for the safety of workers at the nerve center of the American mission in Iraq.

About a dozen shells crashed into the 3.5-square-mile area of central Baghdad about 4 p.m., sending terrified pedestrians racing for the safety of concrete bunkers.

Both the intensity and skill of the attack were noteworthy. The shells, believed to be 122mm, exploded in rapid succession over about a three-minute period.

The blasts were relatively close to one another, suggesting an experienced mortar crew using more than one launcher.

It was unclear whether the rounds were fired by Sunni or Shiite extremists. Both groups operate in areas of the city within rocket and mortar range of the secured complex despite the ongoing Baghdad security crackdown.

U.S. officials would not comment on damage in Wednesday’s attack, citing security.

However, the U.S. Institute of Peace said its office suffered “significant” shrapnel damage though there were no casualties among its staff. The institute sponsored the bipartisan Iraq Study Group, which outlined a plan last December for the withdrawal of most U.S. combat troops by early 2008.

Nine people were wounded in a rocket attack Tuesday, and four Asian contractors were killed in a barrage May 3.

State Department spokesman Tom Casey downplayed the latest attack, saying “it’s been part of the operating environment for our officials there, as well as for other people working there.”

“This is something, unfortunately, that has been a factor and a safety concern for our people since the beginning,” he said. “But certainly we are always looking at what we can do to better protect our staff and our facilities.”

Nevertheless, the recent increase in attacks has raised alarm among American staffers living and working in what had been considered an oasis of safety in the turbulent Iraqi capital. This month, the U.S. Embassy ordered diplomats to wear flak jackets and helmets while outdoors or in unprotected buildings.