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He will preside over a ceremony Wednesday where Gen. Odierno ends more than five years in Iraq and hands over the reins as commander of U.S. forces here to Lt. Gen. Lloyd Austin. Gen. Austin also has served extensively in Iraq, most recently as commander of troop operations in 2008-09.

Under a security agreement between the two nations, all U.S. forces must leave Iraq by the end of 2011. But the Obama administration, sensitive to charges of American abandonment, has directed its diplomats to step into the void and help Iraq’s weak government, economy and other institutions get back on their feet for years to come.

Threats still remain.

Mr. al-Maliki last week put Iraq on its highest level of alert for possible attacks by al Qaeda and Saddam’s former Ba’ath Party loyalists in the days leading up to the U.S. ceremony on Wednesday. An Iraqi intelligence official said suicide bombers are believed to have entered Iraq with plans to strike unspecified targets in Baghdad, the capital.

And on the eve of Mr. Biden‘s arrival, Iraqi police said two mortar rounds landed in the capital’s Green Zone, where the parliament and many foreign embassies are housed behind blast walls, steel gates and barbed wire. The rounds landed near the U.S. Embassy but did not kill or injure anyone, police said.

U.S. Brig. Gen. Ralph Baker said Monday that there had been a marked increase in indirect fire — usually meaning a mortar or rocket — into the Green Zone and the international airport. Over the past two months there have been about 60 indirect fire attacks compared with just a handful in previous months, Gen. Baker said.

The attacks are the work of Shi’ite militias backed by Iran that are trying to portray themselves as driving the American forces from Iraq, Gen. Baker said.

“They’re trying to all claim credit for the U.S. drawdown,” he said.

The vice president’s last trip to Baghdad in July was punctuated by an explosion in the Green Zone; no one was injured in the attack.

All Iraqi security officials spoke on condition of anonymity because they was not authorized to discuss sensitive information with the media.