DA NANG, Vietnam — In the tropical climate of central Vietnam, weeds and shrubs seem to grow everywhere — except here.


Forty years after the United States stopped spraying herbicides in the jungles of Southeast Asia in the hopes of denying cover to Vietcong fighters and North Vietnamese troops, an air base here is one of about two dozen former American sites that remain polluted with an especially toxic strain of dioxin, the chemical contaminant in Agent Orange that has been linked to cancers, birth defects and other diseases.


On Thursday, after years of rebuffing Vietnamese requests for assistance in a cleanup, the United States inaugurated its first major effort to address the environmental effects of the long war.


“This morning we celebrate a milestone in our bilateral relationship,” David B. Shear, the American ambassador to Vietnam, said at a ceremony attended by senior officers of the Vietnamese military. “We’re cleaning up this mess.”


The program, which is expected to cost $43 million and take four years, was officially welcomed with smiles and handshakes at the ceremony. But bitterness remains here. Agent Orange is mentioned often in the news media, and victims are commemorated annually on Aug. 10, the day in 1961 when American forces first tested spraying it in Vietnam. The government objected to Olympics sponsorship this year by Dow Chemical, a leading producer of Agent Orange during the war. Many here have not hesitated to call the American program too little — it addresses only the one site — and very late.


“It’s a big step,” said Ngo Quang Xuan, a former Vietnamese ambassador to the United Nations. “But in the eyes of those who suffered the consequences, it’s not enough.”


Over a decade of war, the United States sprayed about 20 million gallons of Agent Orange and other herbicides in Vietnam, Cambodia and Laos, halting only after scientists commissioned by the Agriculture Department issued a report expressing concerns that dioxin showed “a significant potential to increase birth defects.” By the time the spraying stopped, Agent Orange and other herbicides had destroyed 2 million hectares, or 5.5 million acres, of forest and cropland, an area roughly the size of New Jersey.


Nguyen Van Rinh, a retired lieutenant general who is now the chairman of the Vietnam Association for Victims of Agent Orange/Dioxin, has vivid memories of hearing American aircraft above the jungles of southern Vietnam and seeing Agent Orange raining down in sheets on him and his troops. Plants and animals exposed to the defoliant were dead within days. Many of his troops later suffered illnesses that he suspects were linked to the repeated exposure to Agent Orange, used in concentrations 20 to 55 times that of normal agricultural use.


“I would like to have one message sent to the American people,” Mr. Rinh said in his office, where a large bust of Ho Chi Minh, the wartime leader and icon, stared down from a shelf behind his desk. “The plight of Agent Orange victims continues. I think the relationship would rise up to new heights if the American government took responsibility and helped their victims and address the consequences.”


Those who have worked on the issue say the American government has been slow to address the issue in part because of concerns about liability. It took years for American soldiers who sprayed the chemicals to secure settlements from the chemical companies that produced them. The United States government, which also lagged in acknowledging the problem, has spent billions of dollars on disability payments and health care for American soldiers who came into contact with Agent Orange.


Mr. Shear, the American ambassador, sidestepped a reporter’s question after the ceremony about whether the United States would take responsibility for the environmental and health effects of Agent Orange.


“There is a disconnect between what America has done for its soldiers and what America has done for Vietnam,” said Charles Bailey, the director of the Agent Orange in Vietnam Program, an effort by the Aspen Institute, a nongovernmental organization based in Washington, to reach common ground between the United States and Vietnam on the issue. “I’m sometimes glad I’m not a U.S. diplomat in trying to square that circle.”