Dang Hong Dan is just four years old, but he bears the scars of a war that came to an end 40 years ago.

This week marks four decades since the fall of Saigon and the end of the Vietnam War, a conflict that continues to blight the lives of many.

During the war, 20 million gallons of a herbicide known as Agent Orange and other herbicide and defoliant mixtures were sprayed by the US over vast areas in the centre and south of Vietnam.

dang ky nhan hieu Hong Dan is four years old but he bears the scars of the Vietnam war that came to an end 40 years ago (Unicef/PA)

This was done between 1961 and 1971 to defoliate forests, clear the perimeters of military installations and destroy crops.

It is such an extremely toxic chemical that even a tiny amount may cause various diseases, birth defects and other disabilities.

Dang was born with a cleft lip and a deformed hand and foot, and he is one of many thousands of children who have suffered because their relatives were exposed to the lethal toxin.

Their suffering extends even further as children with disabilities have been among the most stigmatised and excluded people in Vietnam and they face significant challenges in their daily lives - including discrimination, limited access to basic health care, education and other public services.

Many are abandoned and at risk of violence, and are placed in institutions which do not provide adequate care and support.

Due to this inequality, Unicef has been working to strengthen the legal framework in Vietnam that should protect children with disabilities and help provide better services.

These include day centres that allow victims of Agent Orange to come and learn and play, something many of them have been denied previously.

Nguyen Thi Y Duyen, a child protection specialist at Unicef, said: "In Vietnam we have been providing support to the government in several areas.

"Firstly, we support them to strengthen the legal framework. To guide the implementation of the law so the rights of people with disability will be implemented in Vietnam.

"The second area is the capacity building for the government so that they can enhance their skills and knowledge in providing better services to adults and children with disabilities.

"Another big area is to build up the good service models such as respite day care for children.

"Many of them are victims of Agent Orange and they come to play with their friends, receive basic education and rehabilitation.

"The children and their families really appreciate the support, as before that the children had no-one to play with and could not associate or interact with their friends.

"Without the centre, the children would be neglected in the communities because victims of Agent Orange have very severe disabilities and they can't be accepted to the regular schools."

Since the war ended the soil dioxin concentration in the sprayed areas has started to recede to background level.

But sites where the herbicides were stored or handled are still highly contaminated, notably former military air bases.

In these areas, dioxin that entered the soil is migrating slowly, especially through organic materials to which it binds, and is getting into food chains as local people are still being exposed by, for example, eating local fish and fowls.

This means the work charities like Unicef do with those affected by Agent Orange may continue for years to come.