The United States has stepped closer to a total ban on the use and export of cluster bombs with the signing by Barack Obama of a new permanent law that would make it almost impossible for the US to sell the controversial weapons.

The decision was hailed by opponents of the weapons as a "major turnaround in US policy" that overrode Pentagon calls to permit their continued export.

The new legislation, tacked on to a huge budget bill, was passed earlier this week by Congress and now sets such stringent rules for the bombs' use, including a ban on sales where they might be suspected of being used where civilians are present, that it seems unlikely the US could export them again.

Researchers believe the US has transferred hundreds of thousands of cluster munitions, containing tens of millions of unreliable and inaccurate bomblets, to 28 countries. They are regarded by those who campaign against their use as indiscriminate and dangerous to clear up.

Under the new rules, the air and artillery-deployed weapons, which scatter hundreds of bomblets, are required to have a self-destruct failure rate of less than 1%, which few of the US cluster bombs meet, before being cleared for sale.

Despite a temporary ban having been in place, the Pentagon made it clear last year that it was keen to see the export of the weapons again.

The new law comes into force amid growing pressure from Congress for a complete ban on their use, even by the US military. International opposition to cluster bombs, which maim and kill civilians long after they have been fired during conflicts, has been hardening rapidly since the Israeli Defence Forces fired more than 1m into southern Lebanon during the 2006 war with Hezbollah. Last year, a treaty limiting their use was signed by 95 countries, including most of America's Nato allies - but not the US.

The 2008 treaty has in effect dried up the potential market for US-manufactured munitions, but in a policy document put out by the Pentagon last year, the US defence department called for it to be allowed to export cluster bombs for another 10 years.

"This permanent export ban is a major turnaround in US policy," said Steve Goose, arms division director at Human Rights Watch, which has campaigned against cluster munitions. "It brings Washington into closer alignment with international opinion on this terrible weapon.

"The passage of this measure is yet another indication that the president should initiate a thorough review of US policy with respect to cluster munitions. If it is unacceptable for foreign militaries to use these weapons, why would it be acceptable for the US military to use them?"

In July 2008, Robert Gates, the defence secretary, issued a three-page directive spelling out US policy on cluster munitions which described the ordnance as "legitimate weapons with clear military utility". Under that policy, the US will continue to use cluster munitions and, after 2018, will use only munitions with a tested failure rate of less than 1%.

However, in December 2008, a spokeswoman for the Obama transition team said the next president would "carefully review" the treaty banning cluster munitions and "work closely [with] our friends and allies to ensure that the United States is doing everything feasible to promote protection of civilians".

Goose said: "The export ban moves the US one step closer to the position taken by nearly 100 nations, including its closest Nato allies, that have signed the treaty banning cluster munitions. A US decision to sign would certainly signal President Obama's commitment to multilateral diplomacy."

The move was also welcomed by the US Campaign to Ban Landmines. "What is important is the growing awareness that these weapons are beyond the pale," said Lora Lumpe. "This is a significant step towards seeing these weapons banned entirely. The door is closing on cluster munitions. When Georgia and Russia both used them in the war in South Ossetia last year they denied their use, because they realise how unacceptable the use of cluster bombs is now becoming."