Global hunger levels are at their highest for decades with Nigeria, Somalia, South Sudan and Yemen facing the risk of famine and 70 million people in need of food aid, a specialist U.S.-based agency said on Wednesday.

People in 45 countries are unable to feed themselves largely because of conflict, drought and economic instability, the Famine Early Warning Systems Network (FEWS NET) said.

"The combined magnitude, severity, and geographic scope of anticipated emergency food assistance needs during 2017 is unprecedented in recent decades," FEWS NET said in a statement.

In northeastern Nigeria, hit by a seven-year insurgency by Boko Haram terrorists, a study backed by U.N. and other aid agencies suggests famine occurred in 2016 and could be ongoing.

In Yemen and South Sudan, persistent conflict, economic instability, and restricted humanitarian access make famine possible in 2017, FEWS NET said.

In Somalia, failed 2016 rains and a forecast of poor spring rains threaten a repeat of 2011 when famine killed 260,000 people.

One third of the 70 million people needing emergency food aid live in four countries - Yemen, Syria, South Sudan and Malawi - FEWS NET said.

Conflict is disrupting trade and humanitarian access in the first three of these, while Malawi has been hit by poor rains.

An El Niño, or warming of the Pacific Ocean, in 2016, followed by La Niña, characterised by unusually cold temperatures in the equatorial Pacific Ocean, in 2017 are causing hardship across the Horn of Africa and southern Africa.