Floods and droughts are hitting Africa's Sahel region ever more frequently, making it increasingly difficult for vulnerable populations to recover from disasters, a UN aid chief warned Friday, hinting climate change was partially to blame.

"Millions of households ... are extraordinarily vulnerable now after series and series of crises and droughts and floods, in events that are getting closer and closer together," said Robert Piper, who coordinates the UN's humanitarian work in the Sahel region.

Most of the Sahel -- a crisis-prone region spanning Senegal, Gambia, Chad, Burkina Faso, Niger, Mauritania, northern Nigeria, northern Cameroon and Mali -- expects far better rain and harvests this year than in 2012, when a humanitarian emergency was declared as some 18 million people risked going hungry.

But Piper told reporters in Geneva that in the Sahel, "one good agricultural season is not going to turn around the situation," as "vulnerable groups are still reeling from last year's crisis."

Many farmers would likely be forced to use proceeds from this year's harvest to pay off debts from last year, while towering food prices were continuing to hit workers across the region, he said.

While the "rapid and effective" response to the 2012 crisis meant the number of food insecure has gone down to 11 million across the region, five million children are at risk of severe malnutrition, more than last year, he added.

Insecurity and other crises across the region -- such as the conflict in Mali between hardline Islamists and French-led international troops -- have left some 500,000 people living as refugees, and another 400,000 people internally displaced.

That means "900,000 people are on the move," Piper said.

While crises are not new in a region which is home to some of the world's most impoverished countries, they seem to be coming closer and closer together, he stressed.

"The situations is changing on the ground. Climate change and weather patterns are undoubtedly one of the drivers of this," he said, also listing population growth and shifts in demographics with more people moving to cities.

The crises are getting much more difficult to deal with, he said, adding: "We've got to figure out ways of breaking these cycles."