The lack of clean drinking water and sanitation in the world's poorest nations threatens U.N. goals to cut poverty and disease, and raises the risk of conflict, leaders and aid groups said on Wednesday.

Liberian President Ellen Johnson-Sirleaf said the Millennium Development Goal on increasing access to sanitation services had become the the orphan MDG.

The sector is under-discussed, under-prioritized and therefore under-resourced, Johnson-Sirleaf said at an event during a U.N. summit to assess progress on the goals.

She said 26 of 54 African countries were on track to meet the target to halve the number of people without safe drinking water by 2015, but that only six nations looked set to meet the sanitation goal.

In Africa, sanitation is the MDG furthest from being met and globally there are 2.6 billion people living without clean toilets, the WaterAid charity said.

Climate change and increasing pressure on water supplies make progress to meet the water and sanitation goal even more urgent, said Maria Otero, U.S. undersecretary for democracy and global affairs.

We must redouble our efforts. We cannot have a future where children continue to die from water-related diseases, where we have wars over water, she said.

While the world looks set to halve poverty and hunger within five years, the United Nations agrees countries are behind on other goals such as improving education and maternal health, reducing child mortality, combating diseases including AIDS, promoting gender equality and protecting the environment.

U.N. Secretary-General Ban Ki-moon said the water and sanitation millennium goal was vital to the success of other targets.

(Inadequate water and sanitation) increases the likelihood of disease and death, it perpetuates poverty, he said. Water is not only a necessity, it's a human right.