World hunger has risen for the third year in a row, according to a new UN report

World hunger rose in 2017 for a third consecutive year, fuelled by conflict and climate change, the United Nations warned on Tuesday, jeopardising a global goal to end the scourge by 2030.

Hunger appears to be increasing in almost all of Africa and in South America, with 821 million people - one in nine - going hungry in 2017, according to the State of Food Security and Nutrition in the World 2018 report.

Meanwhile, 672 million adults - more than one in eight - are now obese, up from 600 million in 2014.

"Without increased efforts, there is a risk of falling far short of achieving the SDG target of hunger eradication by 2030," the report said, referring to the U.N. Sustainable Development Goals, adopted by member nations in 2015.

It was the third year in a row that global hunger levels have increased, following a decade of declines.

The report's editor Cindy Holleman said increasing variation in temperature; intense, erratic rainfall and changing seasons were all affecting the availability and quality of food.

"That's why we are saying we need to act now," said Holleman, senior economist for food security and nutrition at the Food and Agriculture Organization (FAO).

"Because we're concerned it's not going to get better, that it's only going to get worse," she told the Thomson Reuters Foundation.

Last year, almost 124 million people across 51 countries faced crisis levels of hunger, driven by conflicts and climate disasters, the U.N. said.

Many nations struggling with prolonged conflicts, including Yemen, Somalia, South Sudan and Afghanistan, also suffered from one or more climate shocks, such as drought and floods, the report said.

On Monday, the charity Save the Children warned 600,000 children in war zones could die from extreme hunger by the end of this year as funding shortfalls kick in and warring parties block supplies from getting to the people who need them.

The U.N. said South America's deteriorating hunger situation might be due to the low prices of the region's main export commodities - particularly crude oil.

A lack of food had caused an estimated 2.3 million people to flee Venezuela as of June, the U.N. has said.

Uncertain or insufficient access to food also contributes to obesity because those with limited financial resources may opt for cheaper, energy-dense processed foods that are high in fat, salt and sugar, the report added.

Being deprived of food could also lead to psychological and metabolic changes, said Holleman.

"The emotions and anxieties associated with food deprivation could then lead to disorders and bingeing when you do have food," she said, adding that experiencing this in foetal and early childhood increases the risk of obesity later in life.

Paul Winters, associate vice-president of the International Fund for Agricultural Development (IFAD), said reducing hunger required targeted approaches that went to the roots of chronic poverty.

"That requires having data on where they are, what their limitations are... and making sure we actually do investments that are transformative," he said.

"One of the big concerns is some (donor) countries are shifting much more to humanitarian aid which is important but doesn't build resilience and address the underlying cause."