The decision, announced in a statement released by the U.S. State Department, overturns the position of former President George W. Bush's administration, which had opposed such a treaty on the grounds that national controls were better.
U.S. Secretary of State Hillary Clinton said the United States would support the talks as long as the negotiating forum, the so-called Conference on the Arms Trade Treaty, "operates under the rules of consensus decision-making."
"Consensus is needed to ensure the widest possible support for the Treaty and to avoid loopholes in the Treaty that can be exploited by those wishing to export arms irresponsibly," Clinton said in a written statement.
While praising the Obama administration's decision to overturn the Bush-era policy and to proceed with negotiations to regulate conventional arms sales, some groups criticized the U.S. insistence that decisions on the treaty be unanimous.
"The shift in position by the world's biggest arms exporter is a major breakthrough in launching formal negotiations at the United Nations in order to prevent irresponsible arms transfers," Amnesty International and Oxfam International said in a joint statement.
However, they said insisting that decisions on the treaty be made by consensus "could fatally weaken a final deal."
"Governments must resist US demands to give any single state the power to veto the treaty as this could hold the process hostage during the course of negotiations. We call on all governments to reject such a veto clause," said Oxfam International's policy adviser Debbie Hillier.
The proposed legally binding treaty would tighten regulation of, and set international standards for, the import, export and transfer of conventional weapons.
Supporters say it would give worldwide coverage to close gaps in existing regional and national arms export control systems that allow weapons to pass onto the illicit market.
Nations would remain in charge of their arms export control arrangements but would be legally obliged to assess each export against criteria agreed under the treaty. Governments would have to authorize transfers in writing and in advance.
The main opponent of the treaty in the past was the U.S. Bush administration, which said national controls were better. Last year, the United States accounted for more than two-thirds of some $55.2 billion in global arms transfer deals.
Arms exporters China, Russia and Israel abstained last year in a U.N. vote on the issue.
The proposed treaty is opposed by conservative U.S. think tanks like the Heritage Foundation, which said last month that it would not restrict the access of "dictators and terrorists" to arms but would be used to reduce the ability of democracies such as Israel to defend their people.
The U.S. lobbying group the National Rifle Association has also opposed the treaty.
A resolution before the U.N. General Assembly is sponsored by seven nations including major arms exporter Britain. It calls for preparatory meetings in 2010 and 2011 for a conference to negotiate a treaty in 2012.