DANANG, Vietnam: The United States Thursday began a landmark project to clean up a dangerous chemical left from the defoliant Agent Orange - 50 years after it was first sprayed by American planes on Vietnam's jungles to destroy enemy cover.

Dioxin, which has been linked to cancer, birth defects and other disabilities, will be removed from the site of a former U.S. air base in Danang in central Vietnam. The effort is seen as a long-overdue step toward removing a thorn in relations between the former foes nearly four decades after the Vietnam War ended.

"We are both moving earth and taking the first steps to bury the legacies of our past," U.S. Ambassador David Shear said during the groundbreaking ceremony near the area where a rusty barbed wire fence marks the site's boundary. "I look forward to even more success to follow."

The $43 million joint project with Vietnam is expected to be completed in four years on the 19-hectare (47-acre) contaminated site, located near Danang's commercial airport and an active Vietnamese military base.

Washington has been slow to respond, quibbling for years over the need for more scientific research to show that the herbicide caused health problems and birth defects among Vietnamese. It has given about $60 million for environmental restoration and social services in Vietnam since 2007, but this is its first direct involvement in cleaning up dioxin, which has seeped into Vietnam's soil and watersheds for generations.

Shear added the U.S. is planning to evaluate what's needed for remediation at the former Bien Hoa air base in southern Vietnam, another Agent Orange hotspot.

The remediation begins as Vietnam and the U.S. forge closer ties to boost trade and counter China's rising influence in the disputed South China Sea that's believed rich in oil and natural resources. The U.S. says protecting peace and freedom of navigation in the sea is in its national interest.

The Danang site is closed to the public. Part of it consists of a dry field where U.S. troops once stored and mixed the defoliant before it was loaded onto planes. The area is ringed by tall grass and a faint chemical smell could be detected during a visit to the area Thursday.

The contaminated area also includes lakes and wetlands dotted with pink lotus flowers where dioxin has seeped into soil and sediment over decades. A high concrete wall separates it from nearby communities and serves as a barrier to keep residents from fishing in the tainted water.

The U.S. military dumped some 20 million gallons (75 million liters) of Agent Orange and other herbicides on about a quarter of former South Vietnam between 1962 and 1971, decimating about 5 million acres (2 million hectares) of forest - roughly the size of Massachusetts.

The war ended on April 30, 1975, when northern Communist forces seized control of Saigon, the U.S.-backed capital of former South Vietnam. Some 58,000 Americans died, along with an estimated 3 million Vietnamese. The country was then reunified under a one-party Communist government. Following years of poverty and isolation, Vietnam shook hands with the U.S. in 1995 and normalized diplomatic relations.

The Agent Orange issue has continued to blight the U.S.-Vietnam relationship because dioxin can linger in soils and at the bottom of lakes and rivers for generations, entering the food supply through the fat of fish and other animals.

Although the chemical remains at the Danang site, U.S. officials said Thursday that containment measures implemented in recent years temporarily ended the public health threat to the local community.

In 2007, Vietnamese authorities - with technical assistance from the U.S. Environmental Protection Agency and funding from the nonprofit U.S.-based Ford Foundation - poured a 6-inch (15-cm) concrete slab half the size of a football field over the contaminated area where Agent Orange was mixed. Dioxin is not water-soluble and only spreads when rainfall and runoff move contaminated mud.

Vietnam's Ministry of Defense and the U.S. now plan to excavate 73,000 cubic meters (2.5 million cubic feet) employing technology used to clean superfund sites in the U.S.

Workers will first dig down about 2 meters (6.56 feet). The soil will then be heated to 335 degrees C (635 Fahrenheit) in special containers where the dioxin will break down into oxygen, carbon dioxide and other substances that pose no health risks to humans or animals.

Vietnam's deputy defense minister, Nguyen Chi Vinh, said Thursday he hopes to receive more support from the international community and the U.S. government to help remediate dioxin hotspots elsewhere across the country.

The former U.S. air base in southern Phu Cat has already been identified, but he said many dioxin-contaminated areas in Vietnam have not been adequately assessed.

It is still unclear how much the U.S. will help clean up in the long term and how much it will allocate for people who claim to be Agent Orange victims.