BAGHDAD - Ambassador Ryan Crocker was sworn in as the new top U.S. envoy to Iraq on Thursday, saying he was taking over the “most critical foreign policy mission” facing the United States.

The oath was administered by junior foreign service officer Tina Tran, who had served with Crocker in Pakistan and has been in the Baghdad embassy since last summer.

Crocker, who is fluent in Arabic, told Iraqi employees of the embassy in Arabic that “you are the heroes of the country, in the true meaning of the word.”

Taking up where his predecessor, Zalmy Khalilzad, left off, the 57-year-old Crocker warned Prime Minister Nouri al-Maliki that his government “must take all the necessary steps to unite the country.”

The new ambassador 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 the parliament, Crocker said: “All of this will be very hard but if I thought it was impossible I would not be standing here today.”

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