US Secretary of State John Kerry was to sign the treaty for the US on Wednesday at UN headquarters, joining 86 other countries that have already done so since it was opened for signatures in May, the official said.
"This is a milestone towards ending the flow of conventional arms that fuel atrocities and abuse," said Salil Shetty, secretary general of Amnesty International.
"The tragic situation in Syria underlines the horrific human cost of the reckless global arms trade."
Amnesty said the US supplies arms to 170 countries, and its weapons account for 30 per cent of the world's weapons trade. The treaty, adopted by the UN General Assembly earlier this year, will enter into force after 50 countries have ratified it.