The report by Swedish researchers examined the 27 mandatory U.N. arms embargoes implemented since 1990 on states such as Iraq and Somalia, and groups including al Qaeda.

"Of all the 27 cases ... none has completely stopped the transfer of weapons to the target," said Siemon Wezeman of the Stockholm International Peace Research Institute who wrote the report with Peter Wallensteen, professor of peace and conflict research at Uppsala University in Sweden.

"Always there will be breaches. Does that make arms embargoes useless? No, it doesn't," Wezeman said.

Embargoes were more effective if U.N. peacekeepers were present, preferably with a mandate to enforce the embargo, and if neighboring states cooperated by imposing tight border controls, he said.

Above all, the five permanent members of the U.N. Security Council -- the United States, Russia, China, Britain and France -- as well as the rest of the international community needed to demonstrate political will both to pass the embargo and enforce it strongly once it was in place.

Wezeman contrasted the arms embargo on Iraq after the first Gulf War in 1991, when strong U.S.-led enforcement succeeded in choking most arms supplies to Baghdad, with some West African countries under embargo where "there isn't really anybody who cares about it," so arms flows continue unchecked.

The report looked separately at whether countries targeted by embargoes changed their behavior to comply with Security Council demands and found that usually they did not.

For example in pre-2003 Iraq weapons flows were stopped but it made little difference to Iraqi policy, said Wallensteen.

Only in a quarter of cases did the arms embargo succeed in changing the behavior of the target, the report said.

Wezeman said it was easier to impose arms embargoes on countries such as Liberia which do not have a powerful Security Council protector, than on Sudan, Iran and Myanmar, which are backed by veto-wielding China, Russia or both.

At the same time Sudan or Iran would be more anxious to avoid being branded "pariah states" than small West African nations such as Sierra Leone or Liberia, he said.

"There is a very strong symbolic value in an embargo. It tells you that you've been really, really, really bad," he said. "The Iranian government is sensitive to that kind of pressure."