A global arms trade treaty that aims to regulate the industry and keep weapons out of the hands of criminals has come into effect.
Supporters of the treaty welcomed the development on Wednesday, saying it was long overdue.
However, the Senate in the US, the world's top arms exporter, has not yet ratified it.
The treaty aims to require arms exporters around the world to meet tough export criteria comparable to those in place in the US and other Western democracies.
"Campaigners have been pushing for this moment for a decade," Anna Macdonald, director of the lobby group Control Arms, was quoted by Reuters news agency as saying.
She said the adoption of the treaty marks the "dawn of a new era".
"If robustly implemented, this treaty has the potential to save many lives and offer much-needed protection to vulnerable civilians around the world," Macdonald said.
"It is now, finally, against international law to put weapons into the hands of human rights abusers and dictators."
Of the 130 signatories of the treaty, 60 have ratified it. A total of 50 ratifications were necessary for the pact's entry into force.
The 193-nation UN General Assembly overwhelmingly approved the treaty in April 2013.
The US signed the Arms Trade Treaty in September 2013, but the Senate, the upper house of the congress, has not yet ratified it.
The National Rifle Association, a powerful US firearms lobby, opposes ratification, even though the treaty covers only weapons exports, not domestic gun sales.
Major weapons producers like Russia, China, India and Pakistan have not signed the treaty.
Top arms exporters that have signed and ratified it include the UK, France and Germany.
Ban Ki-moon, UN secretary-general, said in a statement on Tuesday: "[It is] critical that we continue to promote universal participation in the [treaty], by encouraging all states, particularly major arms exporters and importers, to join.
"I call on those states who have not yet done so, to accede to it without delay".
The treaty aims to set standards for cross-border transfers of conventional weapons ranging from small firearms to tanks and attack helicopters
It creates binding requirements for states to review cross-border contracts to ensure weapons will not be used in human rights abuses, violations of humanitarian law or organised crime.