US Secretary of State John Kerry will sign the UN Arms Trade Treaty regulating the $70bn international conventional arms business, in a move that puts the Obama administration at odds with the powerful American gun lobby.
A senior State Department official said President Barack Obama's administration would notify the US Senate of the decision on Tuesday and Kerry would sign the treaty on Wednesday on the sidelines of the annual UN General Assembly in New York.
The arms treaty, which requires ratification by the Senate and has been attacked by America's pro-gun National Rifle Association, would help Western countries press to curtail Russian arms sales to Syria, where President Bashar al-Assad's government has been accused of widespread abuses in more than two years of civil war.
Amnesty International Secretary-General Salil Shetty called Kerry's decision "a milestone towards ending the flow of conventional arms that fuel atrocities and abuse".
The US and 86 other signatory nations "must implement the treaty and bring to an end the supply of weapons to countries where they would be used to commit or facilitate genocide, crimes against humanity, war crimes or other serious human rights violations", Shetty said in a statement.
The UN General Assembly adopted the treaty on April 2 by a vote of 154 for, including the US, three against, and 23 abstentions. The no votes were cast by Iran, North Korea and Syria, UN records showed.
Russia, Syria's main arms supplier, and China abstained in the April UN vote and have not signed the pact.
The NRA, which has opposed the treaty from the start, called the April vote a sad day for the US, the world's top arms exporter, arguing that it undermines American sovereignty and disregards the Second Amendment of the US Constitution, which guarantees citizens the right to bear arms.
The senior State Department official rejected the NRA's characterisation of the treaty, saying the pact's target is "illicit trade in conventional weapons that benefits terrorists and rogue agents".