Legal trade in small arms has grown to at least $8.5 billion a year and if the illict trade was added it would come to more than $10 billion, a weapons research group said Monday.

The Small Arms Survey had estimated the global trade in small arms, munitions and spare parts at more than $4 billion four years ago.

The research group said the market had grown -- because of the wars in Afghanistan, Iraq and elsewhere -- and it also has more accurate information as European countries and the United States are more transparent about deals.

The Small Arms Survey 2012 said the countries which export more than $100 million of small arms were the United States, Italy, Germany, Brazil, Austria, Japan, Switzerland, Russia, France, South Korea, Belgium and Spain.

The top importers with trade of at least $100 million were the United States, Britain, Saudi Arabia, Australia, Canada, Germany and France, the survey found.

Small Arms Survey's Eric Berman told a press conference that legal trading in arms is bigger than the illegal sector "although the illicit trade may do more damage or be more problematic."

"We can clearly say that the two combined would be over $10 billion," he added.

Berman said that munitions accounted almost half the annual figure.

The annual survey said that the market growth was explained by Americans buying more guns and by the wars in Afghanistan and Iraq.

The Small Arms Survey report said that Switzerland, Britain and Romania are the most transparent trading countries and highlighting growing openness by many western countries.

But it added that trade in Africa, Asia, and the Middle East "remains opaque" and "huge gaps" remain in reporting.

"Of particular concern is the lack of information on possible exports to unstable or abusive regimes from China and the Russian Federation, neither of which releases data on exports of pistols, military firearms, light weapons, or light weapons ammunition," said the survey.

"Arms transfers from Iran and North Korea and re-exports from states with large surplus stockpiles, such as Angola, are also poorly understood," it added.

Iran, North Korea and United Arab Emirates came at the bottom of the survey's transparency index. The United States was classed 14th in the 52 countries assessed.

The study highlighted the "significant" percentage of Iranian-origin weapons among those which have been seized in caches in Iraq.

The full report can be accessed at www.smallarmssurvey.org. The Small Arms Survey is based at the Institute of High International Studies in Geneva.

The report was launched as the United Nations started a review conference on the UN Programme of Action on Small Arms and Light Weapons, which aims to strengthen national legislation and controls on illegal small arms production.

The international community must "prevent the flow of illegal small arms into conflict and post-conflict areas and the hands of warlords, traffickers and criminals," UN Secretary General Ban Ki-moon said in a message to the conference.

The 193 UN members failed last month to agree a treaty to regulate the $60 billion a year conventional arms trade. Talks may resume in 2013.