WASHINGTON — The Bush administration was caught off-guard by the first Iraqi-led military offensive since the fall of Saddam Hussein, a weeklong thrust in southern Iraq whose paltry results have silenced talk at the Pentagon of further U.S. troop withdrawals anytime soon.

President Bush last week declared the offensive, which ended Sunday, "a defining moment" in Iraq's history.

That may prove to be true, but in recent days senior U.S. officials have backed away from the operation, which ended with Shiite militias still in place in Basra, Iraqi Prime Minister Nouri al-Maliki possibly weakened and a de facto cease-fire brokered by an Iranian general.

"There is no empirical evidence that the Iraqi forces can stand up" on their own, a senior U.S. military official in Washington said, reflecting the frustration of some at the Pentagon.

The operation faltered in the face of fierce resistance from the Mahdi Army of anti-American Shiite cleric Muqtada al-Sadr, forcing the Iraqi military to turn to U.S. jets and British tanks and artillery to try to dislodge the gunmen from their strongholds.

Having Iraqi forces take a leadership role in combating militias and Islamic extremists was crucial to U.S. hopes of withdrawing more American forces in Iraq and reducing the severe strains the Iraq war has put on the Army and Marine Corps.

Questions remain about how much Bush and his top aides knew in advance about the offensive and whether they encouraged al-Maliki to confront al-Sadr.

A senior U.S. lawmaker and four military officials said Tuesday that the Americans were aware in general terms of the coming offensive, but were surprised by the timing and by the Iraqis' almost immediate need for U.S. air support and other help.

One senior U.S. military commander in Iraq said the Iraqi government originally told the United States about a longer-term plan to rid Basra of rogue elements. But al-Maliki changed the timing, and the nature of the Iraqi operation changed, he said.

"The planning was not done under our auspices at all," the American commander said. The plan changed because "the prime minister got impatient."

There's no evidence, however, that the U.S. tried to dissuade al-Maliki from executing either plan.

Another senior American military official in Baghdad said al-Maliki notified Army Gen. David Petraeus, the U.S. commander in Iraq, and U.S. Ambassador Ryan Crocker less than two days before launching the operation.

"By then it was a done deal," this official said.

Al-Maliki, a Shiite, flew to Basra last week and took personal command of the crackdown, promising he would remain in the city for "a decisive and final battle" to crush the militias.

Fighting eased after al-Sadr called his fighters off the streets Sunday under a deal brokered by Iran. But al-Sadr's fighters refused to surrender their weapons — a development which left the cleric in a position of power and al-Maliki politically battered.

Al-Maliki returned Tuesday to Baghdad, declaring the operation a success although several Basra neighborhoods appeared to remain under militia control.

The apparent misjudgment of the Iraqi security forces' capabilities and the strength of al-Sadr's Mahdi Army militia, as well as the revived political controversy over the war, come at an inopportune moment for the White House.

Petraeus and Crocker are due to testify to Congress next week about the strategy in Iraq now that the 30,000 additional troops Bush ordered there are being withdrawn.

In the larger sense, "this is a reminder that nothing has changed," said a senior State Department official, who also wasn't authorized to speak publicly.

As if to underscore that point, Britain announced Tuesday that it's freezing plans to withdraw 1,500 of its 4,000 remaining troops from southern Iraq due to the failure of the Iraqi offensive to crush Shiite militias.

Bush already has signaled that, after the Petraeus-Crocker report, he'll order a pause in further drawdowns of U.S. troops in Iraq below about 140,000, which is slightly more troops than were in Iraq before the U.S. buildup began.

The Pentagon planned to reduce troop levels by one brigade a month, thin out its presence in Iraq and lean more heavily on Iraqi forces. But the Basra offensive has some in the U.S. military fretting that Iraq's forces, while better than they were six months ago, cannot fully defend their communities.

Some say that Iraqi security forces are entangled in the intra-Shiite battle for power in southern Iraq. The Iraqi forces that al-Maliki sent to Basra contained a large number of one-time fighters in the Badr Organization, the armed wing of the Islamic Supreme Council of Iraq, which vies for power with al-Sadr's Mahdi Army.

"We're not going to stop the tensions between the Shiite camps. Those were there all along; we've just seen them emerge," said retired Army Lt. Gen. William Odom, former director of the National Security Agency and a longtime war critic, during a conference call.

Indeed, violence began rising in places where the U.S. military drew down its forces. The first brigade left in December from the volatile Diyala province in northeast Iraq.

The U.S. military moved two battalions out of Baghdad to cover parts of Diyala and Mosul, a Sunni stronghold in northern Iraq, according to the military.

Figures compiled by the ministries of health, interior and defense showed that at least 1,720 Iraqi civilians were killed in politically motivated violence in March. That was up sharply from the 953 figure for February.

Figures tabulated by The Associated Press from police and U.S. military reports put the March death toll as of Monday at 1,247 — nearly double the February figure and the biggest monthly toll since August, when 1,956 people died violently.

Information from The Associated Press is included in this report.