The U.S. ambassador to Iraq said Friday that American diplomats will start moving into the new embassy in Baghdad at the end of May, and should be safer in the more fortified complex inside the so-called Green Zone, which has been hit repeatedly by insurgent rocket fire.

The envoy, Ryan Crocker, also defended the work of private security contractors like Blackwater USA, which has been accused of using unnecessary deadly force while protecting U.S. diplomats in Baghdad, as "absolutely essential" to the functioning of embassy staff.

Crocker said buildings at the massive, Vatican-sized embassy compound have been completed, although they have not yet all been certified for permanent occupancy, and that the long-delayed move-in, originally scheduled for last fall, would begin at the end of next month after a spike in rocket attacks on the Green Zone that have killed several Americans.

"It's been a difficult few weeks, rockets are bouncing off your buildings, and maintaining focus can be an occasional challenge," he told reporters.

"We will begin moving into the new embassy — some of the office space and the apartments — probably the end of next month, the beginning of June, so that will certainly improve quality of life and provide some added protection," Crocker said.

The recent jump in insurgent attacks has led the State Department to order embassy personnel not to leave reinforced buildings and to wear helmets and body armor if they must go outside. And, a shortage of space in fortified areas has forced some diplomats to sleep temporarily at the new embassy despite the lack of final occupancy approvals.

"We worry a lot less about formal safety certifications and a lot more about ensuring people have a place to sleep where rockets couldn't get at them," Crocker said.

The $600 million embassy project has been beset by construction, logistical and security hitches that have caused major delays in its planned Sept. 2007 opening date and angered some lawmakers.

At the same time, the private guards who provide security to U.S. diplomats in Iraq have come under intense criticism following an incident in which Blackwater employees are alleged to have killed Iraqi civilians.

The State Department last week renewed Blackwater's contract despite the fact that an FBI investigation into that incident is still under way.

Crocker said he and his team would not able to do their jobs if they did not have such protection.

"The challenges of getting the nation's business done in Iraq are pretty substantial," he said. "We have to function in conditions that would in most places have us pretty much in a stand-down. But this is the nation's most critical work, and it has to go on, and security contractors like Blackwater are absolutely essential to this effort."