BAGHDAD (AP) — Vice President Joe Biden returned to Iraq Monday to mark the formal end to U.S. combat operations and push the country's leaders to end a six-month stalemate blocking formation of a new government.

He came to preside over a military change-of-command ceremony on Wednesday. The event will signal a shift toward a greater U.S. diplomatic role as the military mission dwindles seven years after the American invasion that toppled Saddam Hussein.

Biden tried to reassure Iraqis on the transition.

"We're going to be just fine. They're going to be just fine," he said during a photo opportunity at the U.S. Embassy. He was flanked by Gen. Ray Odierno, Ambassador Jim Jeffrey and Marine Gen. James Mattis, the new leader of the U.S. Central Command.

The Sept. 1 ceremony also marks the start of the so-called "Operation New Dawn" — symbolizing the beginning of the end of the American military's mission in Iraq since invading in March 2003.

Just under 50,000 U.S. troops remain in Iraq — down from a peak of nearly 170,000 at the height of the 2007 military surge that is credited with helping turn the tide in Iraq as it teetered on the brink of civil war. Additionally, U.S. troops no longer will be allowed to go on combat missions unless requested and accompanied by Iraqi forces.

Underscoring the shift, officials said Biden will make a new appeal to Iraqi leaders, including Prime Minister Nouri al-Maliki and political archrival former premier Ayad Allawi, to end the political deadlock and seat a new government. March 7 parliamentary elections left Iraq without a clear winner, and insurgents have exploited the uncertainty to hammer Iraqi security forces in near-daily attacks.

In a daylong meeting Tuesday, Biden will "urge Iraqi leaders to conclude negotiations on the formation of a new government," the White House said in a statement.

Allawi heads the secular, Sunni-dominated Iraqiya political coalition that narrowly denied al-Maliki a win in the March vote. Both al-Maliki and Allawi want to be prime minister, and U.S. diplomats have encouraged a power-sharing agreement between them to control a majority of parliament and win the right to choose the new government's leaders.

So far, neither man has backed down, creating a political impasse and leading to back-room jockeying by hard-line Shiite groups for a larger share of power.

The White House said Biden also plans to sit down with Iraqi President Jalal Talabani, Sunni Vice President Tariq al-Hashemi, Shiite Vice President Adel Abdul-Mahdi, and Shiite cleric Ammar al-Hakim, leader of the Iranian-backed Supreme Islamic Iraqi Council.

Tony Blinken, Biden's top national security adviser, said the delay in seating a new government has slowed U.S. pressure on the United Nations to lift sanctions on Iraq that have been in place since the 1991 Kuwaiti invasion.

Still, Blinken said the U.S. is "determined to build a long-term partnership" with Iraq.

"We're not disengaging from Iraq," Blinken told reporters. "And even as we draw down our troops, we are ramping up our engagement across the board."

It was the vice president's sixth trip to Iraq since he was elected.

He will preside over a ceremony Wednesday where 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. 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.

Al-Maliki last week put Iraq on its highest level of alert for possible attacks by al-Qaida and Saddam's former Baath 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 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 last two months there have been about 60 indirect fire attacks compared to just a handful in previous months, Baker said.

The attacks are the work of Shiite militias backed by Iran trying to portray themselves as driving the American forces from Iraq, 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.