BAGHDAD — U.S. Ambassador Ryan Crocker was sworn in as the new top U.S. envoy to Iraq today, saying that he was taking over the "most critical foreign policy mission" facing his country. A series of bomb blasts in Baghdad and its suburbs underlined how tough his job will be.

Just hours before Crocker addressed embassy employees in the heavily fortified Green Zone, a bomb planted under a parked car tore through an outdoor Baghdad market in the mixed Al-Bayaa neighborhood, killing three and wounding 26.

About the same time, a car bomb went off near a Shiite mosque in the restive town of Mahmoudiyah, 20 miles south of Baghdad, killing six people and wounding 19.

The mosque and four adjacent stores were slightly damaged, according to a police officer at the scene who spoke on condition of anonymity because he is not authorized to speak to the media.

Crocker, a fluent Arabic speaker, used that language when he told the embassy's Iraqi employees, "You are the heroes of the country, in the true meaning of the word."

Taking up where his predecessor, Zalmay Khalilzad, left off, the 57-year-old Crocker warned Shiite Prime Minister Nouri al-Maliki that his government "must take all the necessary steps to unite the country." He left no doubt of his commitment to the Bush administration's policy in Iraq, which is under withering attack in the Democrat-controlled Congress.

"President Bush's policy is the right one. There has been progress; there is also much more to be done," he said.

After listing challenges faced in the coming months, including shepherding benchmark legislation through parliament, Crocker said: "All of this will be very hard but if I thought it was impossible I would not be standing here today."

Sitting next to Crocker at the ceremony in the U.S. Embassy, housed in Saddam Hussein's former Republican Palace in the Green Zone, was U.S. commander in Iraq Gen. David Petraeus, who only recently took over military operations in the country.

"I also recognize our brave military colleagues who risk their lives each day to secure a better tomorrow for the Iraqi people, thereby serving the interests of us all. I look forward to working with Gen. Petraeus and all of you in the months ahead. And, general, I promise you a full unity of effort."