BAGHDAD - The United States yesterday inaugurated its largest embassy - a fortresslike compound in the heart of the Green Zone and the most visible sign of what U.S. officials call a new chapter in relations between America and a more sovereign Iraq.

U.S. Marines raised the American flag over the adobe-colored buildings, which sit on a 104-acre site and have space for 1,000 employees. The $700 million compound is more than 10 times the size of any other U.S. Embassy in the world.

"Iraq is in a new era and so is the Iraqi-U.S. relationship," Ambassador Ryan Crocker proclaimed.

In perhaps an unintended sign of the new relationship, Prime Minister Nouri al-Maliki did not attend yesterday's ceremony because he was traveling in Iran, a country the United States has accused of aiding and arming Iraqi extremists.

Explaining the opening of such a large embassy three years before the United States must finish withdrawing its 146,000 troops from Iraq, Crocker said it was vital the United States remain involved in nonmilitary ways. "I think we have seen a tremendous amount of progress," he said before the ceremony, "but the development of this new Iraq is going to be a very long time in the making, and we need to be engaged here."

He said Baghdad was looking to the West for the first time since the army's 1958 revolution that toppled Iraq's monarchy and set the stage for the ascendance of the Baath party, which dominated Iraq until the 2003 invasion.

"Iraq has defined itself in general hostility to the West and the United States," Crocker said. "You now have a fundamentally different state and society taking shape that values those relations, that values those contacts, that wants its children educated in American and other Western universities. And we need to be there as a partner to ensure that those relationships are solidly built and well-maintained."

The inauguration of the embassy came just days after a security agreement between Iraq and the United States took effect, replacing a U.N. mandate that gave legal authority to the United States and other foreign troops to operate in Iraq. Under the new pact, U.S. troops will no longer conduct unilateral operations and will act only in concert with Iraqi forces. They must also leave major Iraqi cities by June and the entire country by 2012.

Crocker said that since the 2003 invasion, "perhaps no single week has been more important than this past week. On Dec. 31 we left the Republican Palace."

On that day, U.S. diplomats and military officials moved into the new embassy and vacated Saddam Hussein's Republican Palace, which they occupied when they captured Baghdad in April 2003. The palace will now seat the Iraqi government.

The new embassy's exact dimensions are classified, but it is said to be six times larger than the U.N. complex in New York and more than 10 times the size of the new U.S. Embassy in Beijing, which at 10 acres is America's second-largest mission.

Reinforced concrete surrounds the new compound, which provides housing for hundreds of staff who had been living in makeshift quarters with aluminum walls that provided little protection.

Iraqi President Jalal Talabani praised President Bush's decision to invade Iraq and topple Hussein. "This building is not only a compound for the embassy, but a symbol of the deep friendship between the two peoples of Iraq and America," he said.

But as U.S. and Iraqi officials lauded progress in the country, Baghdad was rocked by a second day of violence that saw four car bombs explode in various parts of the city, killing four and wounding 19. On Sunday, a suicide bomber killed at least 38 people at a Shiite shrine just four miles north of the new embassy.

Although violence has plummeted in the last year, with attacks dropping from an average 180 a day to just 10, horrific bombings still plague the capital.