BAGHDAD, July 15 — An American general directing a major part of the offensive aimed at securing Baghdad said Sunday that it would take until next spring for the operation to succeed, and that an early American withdrawal would clear the way for “the enemy to come back” to areas now being cleared of insurgents.

Maj. Gen. Rick Lynch, commanding 15,000 American and about 7,000 Iraqi troops on Baghdad’s southern approaches, spoke more forcefully than any American commander to date in urging that the so-called troop surge ordered by President Bush continue into the spring of 2008. That would match the deadline of March 31 set by the Pentagon, which has said that limits on American troops available for deployment will force an end to the increase by then.

“It’s going to take us through the summer and fall to deny the enemy his sanctuaries” south of Baghdad, General Lynch said at a news briefing in the Iraqi capital. “And then it’s going to take us through the first of the year and into the spring” to consolidate the gains now being made by the American offensive and to move enough Iraqi forces into the cleared areas to ensure that they remain so, he said.

The general spoke as momentum is gathering in Congress for an early withdrawal date for the 160,000 American troops, as well as an accelerated end to the troop buildup, which have increased American combat casualties in the past three months to the highest levels of the war. In renewed debate over the past week, Congressional opponents of the war have demanded a withdrawal deadline, with some proposing that Congress use its war-appropriations powers to end the troop increase much sooner, possibly this fall.

General Lynch, a blunt-spoken, cigar-smoking Ohio native who commands the Third Infantry Division, said that all the American troops who began an offensive south of Baghdad in mid-June are part of the five-month-old troop buildup, and that they were making “significant” gains in areas that were previously enemy sanctuaries. Pulling back before the job was completed, he said, would create “an environment where the enemy could come back and fill the void.”

He implied that an early withdrawal would amount to an abandonment of Iraqi civilians who he said have rallied in support of the American and Iraqi troops, and would leave the civilians exposed to renewed brutalities by extremist groups. “When we go out there, the first question they ask is, ‘Are you staying?’ ” , he said. “And the second question is, ‘How can we help?’ ”. He added, “What we hear is, ‘We’ve had enough of people attacking our villages, attacking our homes, and attacking our children.’ ”

General Lynch said his troops had promised local people that they would stay in the areas they had taken from the extremists until enough Iraqi forces were available to take over, and said this had helped sustain “a groundswell” of feeling against the extremists. He said locals had pinpointed hideouts of Al Qaeda in Mesopotamia, an extremist group that claims to ties to Osama bin Laden, that have been used to send suicide bombers into Baghdad, helped troops locate 170 large arms caches. The general also said the locals had founded new neighborhood patrol units called “Iraqi provincial volunteers” that supplied their own weapons and ammunition.

The general declined to be drawn into what he called “the big debate in Washington” over the war, saying American troops would continue to battle the enemy until ordered to do otherwise. But he made it clear that his sympathies lay with the Iraqis in his battle area, covering an area about the size of West Virginia, mostly between the Tigris and Euphrates rivers, that extends about 80 miles south of Baghdad and includes four of Iraq’s 18 provinces. The offensive he commands is part of a wider push by American and Iraqi forces in all the areas surrounding Baghdad, and in the capital itself, that began in February.

“What they’re worried about is our leaving,” he said. “And our answer is, ‘We’re staying’, because my order from the corps commander is that we don’t leave the battle space until we can hand over to the Iraqi security forces.” To hold recent gains, he said, would require at least a third more Iraqi troops than he now has, and they would have to come from other battle areas, or from new units yet to complete their training. “Everybody wants things to happen overnight, and that’s not going to happen,” he said. General Lynch’s outspoken approach contrasted with the more cautious remarks made recently by other senior American officers, including the top American commander here, Gen. David H. Petraeus. General Petraeus has said in recent interviews that the troop buildup had made substantial gains. But he has declined to say whether he will urge a continuation of it when he returns to Washington by mid-September to make a report on the war to President Bush and Congress that was made mandatory by war-appropriations legislation this spring.

General Lynch said he was “amazed” at the cooperation his troops were encountering in previously hostile areas. He cited the village of Al Taqa, near the Euphrates about 20 miles southwest of Baghdad, where four American soldiers were killed in an ambush on May 12 and three others were taken hostage. One of the hostages was later found dead, leaving two soldiers missing.

Brig. Gen. Jim Huggins, a deputy to General Lynch, said an Iraqi commander in the area had told him on Saturday that women and children in the village had begun using plastic pipes to tap on streetlamps and other metal objects to warn when extremists were in the area planting roadside bombs and planning other attacks. “The tapping,” General Huggins said, was a signal that “these people have had enough.”

General Lynch also challenged another argument often made by American lawmakers who want to end the military involvement in Iraq here soon: that Iraqi troops have ducked much of the hard fighting, and often proved unreliable because of the strong sectarian influence exercised by the competition for power between Shiite, Sunni and Kurdish political factions. “I don’t know,” he said, how American war critics had concluded that the new American-trained Iraqi Army was not up to the fight. “I find that professionally offensive,” he said, after noting that there were “many Iraqi heroes” of the fighting south of Baghdad. “They’re competent. There’s just not enough of them,” he said.

General Lynch said that he and other American commanders were worried that extremist groups under attack by the additional American forces might retaliate with a spectacular, focused attack on American troops aimed at tipping the argument in Washington in favor of withdrawal. He cited what happened in South Vietnam in January 1968, when coordinated attacks by enemy troops, including one on the American embassy in Saigon, helped push President Lyndon B. Johnson into abandoning attempts to win the war. “We’re concerned about some kind of Tet offensive that’s going to affect the debate in Washington,” the general said.