Washington signaled Friday a looming showdown at the United Nations when 193 countries will try again to hammer out the first international treaty on the $70 billion global arms trade.
The first bid to draw up an arms trade treaty came close to agreement but deadlocked in July after the United States asked for more time to pour over the draft text despite years of preparatory work.
US Secretary of State John Kerry said the administration of President Barack Obama supported a treaty, due to be debated at a 10-day conference at the UN starting Monday.
But he warned that the US "would only be party to one that addresses international transfers of conventional arms solely and does not impose any new requirements on the US domestic trade in firearms or on US exporters."
The Obama administration also opposes any text that goes against existing US law, including the Constitution and Americans' rights to bear arms under the Second Amendment.
One of the major sticking points is a call from many governments and activists to impose regulations on sales of ammunition -- a move long opposed by the United States, the world's leading arms producer.
"Ammunition is fundamentally different from weapons, and its inclusion would present a variety of practical and implementation difficulties," a US official told AFP.
Kerry stressed that "the United States is steadfast in its commitment to achieve a strong and effective Arms Trade Treaty that helps address the adverse effects of the international arms trade on global peace and stability."
But he insisted the treaty must recognize "that each nation must tailor and enforce its own national export and import control mechanisms."
"We support a treaty that will bring all countries closer to existing international best practices, which we already observe, while preserving national decisions to transfer conventional arms responsibly," he added.
Amnesty International's North America deputy executive director, Frank Jannuzi, argued that if US support is only "lukewarm," then other states "will take great comfort and will exploit that lack of leadership to opt out."
"At a minimum we need to capture within this treaty's provisions the export of munitions that allow these weapons to kill," he said.
"Amnesty's principle concern is that an AK-47 can last for three decades and without ammunition, it's a club."
A treaty that fails to regulate the control of ammunition exports and imports would "have an enormous loophole and its effectiveness will be tremendously undermined," Jannuzi said.
Ironically, if the United States fails to sign any arms trade treaty, it could find itself among such pariah states as Iran and North Korea, which have also voiced opposition to such a pact.
Nobel laureates, top church leaders and former generals have also sent letters to Obama urging the administration to get behind a treaty "with teeth."
One letter signed by retired top generals pointed to the abundance of arms in Afghanistan. "The relative ease that terrorists and insurgents receive weapons from the under-regulated international weapons trade directly puts our men and women in uniform under threat," they wrote.
Church leaders urged Obama to show "global moral leadership" to end the arms epidemic and "make it much harder for terrorists and warlords to get their hands on weapons and ammunition."
And 18 Nobel laureates, including former US president Jimmy Carter and South Africa's Archbishop Desmond Tutu, reminded Obama of his clarion call that "our actions matter" when he received the Nobel Peace Prize in 2009.
They said they could "not accept the deaths of hundreds of thousands... gunned down each year, with millions left maimed and traumatized."
"The absence of effective, legally binding international rules regulating the arms trade represents a colossal failure of the international community," the group added.