A new survey by Transparency International (TI), an anti-graft organisation, also rates Nigeria, Chad, Myanmar, Azerbaijan and Paraguay among the world's most corrupt nations.
Finland is ranked the least corrupt country in the world.
Other nations with very low levels of corruption are New Zealand, Denmark, Iceland, Singapore, Sweden and Switzerland, TI says.
The study, which reflects the perceptions of business people and country analysts, draws on surveys conducted by independent institutions between 2002 and 2004.
Robin Hodess, director of policy and research at TI, told Aljazeera.net there was a direct correlation between poverty and corruption.
She said the more corrupt countries in the world - which are dominated by developing nations - are also characterised by political instability.
"Cleaner countries have stronger institutions and are more open," she said.
"There is more wealth and more freedom in these societies. And there also seems to be a direct connection between media freedom and less corruption."
"Cleaner countries have stronger institutions and are more open. There is more wealth and more freedom in these societies. And there also seems to be a direct connection between media freedom and less corruption"
Moreover, TI estimates the amount lost due to bribery in government acquisitions is at least $400 billion a year worldwide.
Its study says developing countries with large oil sectors are particularly prone to corruption.
"Corruption in large-scale public projects is a daunting obstacle to sustainable development, and results in a major loss of public funds needed for education, healthcare and poverty alleviation, both in developed and developing countries," said TI chairman Peter Eigen.
"Oil-rich Angola, Azerbaijan, Chad, Ecuador, Indonesia, Iran, Iraq, Kazakhstan, Libya, Nigeria, Russia, Sudan, Venezuela and Yemen all have extremely low scores.
"In these countries, public contracting in the oil sector is plagued by revenues vanishing into the pockets of Western oil executives, middlemen and local officials."
TI said Western governments should oblige their oil companies to publish what they pay in fees and royalties, as well as other payments to host governments and state oil companies.
TI says Iraq's future depends on
the transparency of the oil sector
Eigen added the future of Iraq depended on transparency in the oil sector.
"The urgent need to fund post-war construction heightens the importance of stringent transparency requirements in all procurement contracts," he said.
"Without strict anti-bribery measures, the reconstruction of Iraq will be wrecked by a wasteful diversion of resources to corrupt elites."
However, non-governmental organisations say the world's richer nations are partly responsible for the plight of the world's poor because of colonialism, conquest and the effects of free trade policies and globalisation.
They also contend the developed world's insatiable thirst for oil is fuelling political instability in poorer nations.
Last year, UK-based Christian Aid released a report saying the presence of oil in a developing country makes life worse, not better, for the people who live there.
The report said oil was the key ingredient in "a lethal cocktail of greater poverty for the vast majority of the population, increased corruption, a greater likelihood of war or civil strife, and dictatorial or unrepresentative government".