Seven of the world's top 10 "failed states" are in Africa, according to a study published in the United States.

The world's "most vulnerable" nation, according to the annual Failed States Index, is Somalia, followed by Chad, Sudan, Zimbabwe and the Democratic Republic of Congo. The next African countries on the list, where conditions are a little better than in Afghanistan and Iraq, are the Central African Republic and Guinea.

Other African countries the index considers among the top 20 failed states are Cote d'Ivoire, Kenya, Nigeria, Ethiopia and Niger.

The index is published annually by Foreign Policy magazine and the Fund for Peace, both based in Washington, DC.

Foreign Policy says in its latest edition that the 10 most unstable countries "are a sadly familiar bunch. Shattered Somalia has been the No. 1 failed state for three years running, and none of the current top 10 has shown much improvement, if any, since... [the magazine and fund] began publishing the index in 2005."

It said Somalia "saw yet another year plagued by lawlessness and chaos, with pirates plying the coast while radical Islamist militias tightened their grip on the streets of Mogadishu."

"Already isolated Sudan saw its dictator, Omar Hassan al-Bashir, defy an arrest warrant from the International Criminal Court and the war-ravaged Democratic Republic of the Congo once again proved itself a country in little more than name," the magazine added.

The report said that even good news for the troubled states came tempered by hard facts. “A coalition government in Zimbabwe whipped history's second-worst bout of hyperinflation, fostering the country's first year of positive growth in more than a decade... But Robert Mugabe's security goons still rule Harare unchecked..."

The publishers say the index draws on 90,000 publicly available sources to analyse 177 countries and rate them on 12 metrics of state decay. These include refugee flows, economic implosion, human rights violations and security threats.