JOHANNESBURG, South Africa — As hunger in the Horn of Africa dramatically worsens, the United Nations on Wednesday added three new regions of Somalia to the list of areas it says are stricken by famine.

More than 12 million people are facing starvation, with children particularly vulnerable. The U.N. last month declared that two regions of Somalia were suffering from famine, and it said Wednesday that the famine was likely to spread across most of Somalia in coming months, as well as parts of Kenya, Uganda and Ethiopia.

Somalia is struggling with its worst drought in 60 years, and 3.7 million Somalis are in crisis, mainly in the south — creating Africa’s most serious hunger crisis in two decades. Refugee camps in the capital, Mogadishu, now are affected as well, U.N. agencies said.

Shocking images of the famine have resulted in a spike in aid in the past two weeks, after donors’ earlier sluggish response, but violence in the south of the country has limited humanitarian agencies’ access.

The U.N. is seeking to raise $1 billion to address the crisis.

“The current situation represents the most severe humanitarian crisis in the world today and Africa’s worst food security crisis since Somalia’s 1991/92 famine,” said a statement Wednesday by the U.S.-funded Famine Early Warning Systems Network. It called for a massive global response to prevent more deaths and social collapse

Increases in food prices in coming months are expected to exacerbate the crisis.

About 860,000 people have trekked out of Somalia, often leaving dead children on the way, hoping to find food in neighboring countries. Another 1.5 million people have fled drought-ridden areas of Somalia for other parts of the war-torn nation.

Somalia has endured two decades without a functional government, and entrenched clan warfare. Multiple peace deals and efforts to establish a government have failed.

The current crisis is exacerbated by fighting between Somalia’s weak transitional government and the extreme Islamic militia al-Shabab.