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WASHINGTON - U.S. commanders in Iraq are developing contingency plans to send more American forces to the country if the situation worsens, and administration officials said Monday that the new surge of violence by Shiites represents a challenge to their plans to turn over power in less than 90 days.

President Bush, speaking in Charlotte, N.C., said he intends to stick to the June 30 date for giving control of the country to an interim Iraqi government, even while conceding that the new government's structure has not been settled. He vowed that the violence, which he said is being instigated by Muqtada al-Sadr, a young Shiite cleric, will be put down, saying, "We just can't let it stand."

Bush appeared eager Monday to dispel any thought that the new wave of attacks on U.S. forces, in which Shiites as well as Sunnis have now joined, will shake his resolve.

"If they think that we're not sincere about staying the course, many people will not continue to take a risk toward . . . freedom and democracy," he told reporters.

Gen. John Abizaid, senior commander in the Middle East, has asked for contingency plans for increasing the number of troops in Iraq. No decision has been made to supplement the 134,000 troops currently in Iraq.

In Iraq, U.S. administrators declared Sadr an "outlaw" Monday and announced a warrant for his arrest, heightening a confrontation after battles between his supporters and coalition troops killed at least 52 Iraqis and nine coalition troops, including eight Americans.

Sadr's armed followers all but shut down the holy city of Najaf and skirmished in Baghdad with U.S. troops in the second day of deadly rioting across Iraq.

American officials would not say when they would move to arrest the cleric, who was holed up in the main mosque in Kufa, south of Baghdad, guarded by armed supporters who control the city.

"We don't fear death, and martyrdom gives us dignity from God," said Sadr, a 30-year-old firebrand who has frequently denounced the U.S. occupation.

"(The Americans) have the money, weapons and huge numbers, but these things are not going to weaken our will because God is with us," he said in a statement sent to the Arab TV station Al-Jazeera, which provided a copy to the Associated Press.

Meanwhile, U.S. troops surrounded the city of Fallujah, west of Baghdad, poised for a major operation in response to the grisly slaying and mutilation of four American civilians by insurgents there last week. A Marine was killed Monday in the Fallujah area, the military said, without providing details.

The showdown with Sadr threatened to heighten tensions with Iraq's Shiite Muslim majority at a time when U.S. troops are burdened by the Sunni guerrillas' bloody insurgency. But American officials apparently hope the Shiite public, many of whom distrust Sadr, will not rally around the cleric.

Brig. Gen. Mark Kimmitt said the potential for violence depends on "whether (Sadr) decides to come peacefully or whether he decides to come not peacefully; that choice is the choice of Mr. Muqtada al-Sadr."

Several hundred of Sadr's armed militiamen control Kufa, holding its police station and blocking a road leading to the main mosque.

Senior military and White House officials said that the attacks by Sadr's forces did not represent a full-scale Shiite uprising or portend a broader civil war.

Still, a senior military official described the violence as part of "a power grab at a very difficult time" and one that administration officials said could interfere with their efforts to draw down the number of U.S. troops as the presidential election approaches.

Hundreds of thousands of Shiite pilgrims will be traveling in the days ahead to Iraq's holy cities of Najaf and Karbala, and senior military officials said the United States want to avoid "doing something stupid that would put more people into the camp of anti-coalition forces."

Democrats, led by Bush's presumptive opponent in the presidential race, Sen. John Kerry, seized the moment to question the underlying logic of Bush's Iraq policy.

"We can't allow this to continue," Kerry told reporters Monday.

"There has to be a political, diplomatic solution which, regrettably, this administration seems stubbornly determined to avoid."

He called the absence of Arab neighbors as part of the stabilization force "staggering," adding that "all have a major stake in not having a failed Iraqi state, no matter how they feel about our getting there."

"I think the president owes it to the American people to explain who we're turning over sovereignty and how on June 30th, and what is the security plan for after June 30th," Kerry concluded.

But he left it to Sen. Edward Kennedy, his old friend and occasional campaign surrogate, to compare the events in Iraq to the Vietnam War four decades ago.

Although Kerry said he would not make analogies to past conflicts, Kennedy said, "Iraq is George Bush's Vietnam, and this country needs a new president."

Sen. Carl Levin, D-Mich., senior Democrat on the Senate Armed Services Committee, joined a prominent Republican, Sen. Dick Lugar, R-Ind., in calling on the administration to leave the door open to the possible postponement of the transfer.

In a telephone interview Monday, Levin said he had spoken to Republicans as well as Democrats who shared his view that the administration should be open to delaying the transfer, particularly if prominent Iraqis outside the current Governing Council favor such a postponement.

"I don't want to see President Bush in a position where flexibility will be seen as a political defeat," Levin said.



Associated Press contributed to this article.