US President Barack Obama is being urged to address the ongoing health impacts caused by Agent Orange when he visits Vietnam later this week.

The US military sprayed around 12 million gallons of the toxic herbicide across the country during the war 40 years ago.

It contained dioxin, one of the most poisonous chemicals ever known.

Campaigners want the President to visit some of the tens of thousands of people over three generation that Vietnamese authorities say are still living with the effects of exposure to the herbicide.

The US government already pays compensation to American veterans and their families, for numerous health problems linked to Agent Orange exposure.

But US officials have consistently denied a connection between contact with the defoliant, which was used to destroy forest that provided cover for the Viet Cong, and similar health issues experienced by people in Vietnam.

The President of the Da Nang Association for Victims of Agent Orange, Nguyen Thi Hien, said President Obama should visit one of the many victim centers around the country to understand the perceived injustice.

"If you compensate US veterans it means you recognize and acknowledge that those who served in the war are Agent Orange victims, why can't you recognize justice for the victims here? We are all victims of the same thing - it's not fair" she said.

The US has, however, been stepping up its assistance to Vietnam in recent years.

The first phase of a multi-million dollar US-funded dioxin decontamination project at Da Nang airbase has now been completed, with around 45,000 cubic metres of soil cleaned through heat processing.

Vietnam's Deputy Prime Minister Vu Duc Dam and US Ambassador Ted Osius both gave speeches at the ceremony praising the "comprehensive partnership" and "friendship" that now exists between the two countries.

But when asked by Sky News whether the time had come for the US to explicitly address the health impacts of Agent Orange in Vietnam, Ambassador Osius ducked the question and hurried to his vehicle.

There is an expectation that President Obama may announce funding for an even bigger decontamination project during his visit, likely at the Bien Hoa airbase outside Ho Chi Minh city, where Agent Orange was stored during the war.

That will certainly be welcomed, but Vietnamese politicians have made clear there is more that could be done.

"I would like to see President Obama make a firmer declaration about how Vietnam and the US can overcome the consequences of war.

"We need that in order to close the door to the past and open a door to future cooperation" said Vietnam's Vice Minister of National Defence, Senior Lieutenant General Nguyen Chi Vinh.

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