* U.S. to sign U.N. arms trade pact it voted for in April
* American gun lobby has opposed treaty for years
* Rights group sees tool to press Russia on Syria arms (Adds senior State Department official)
By Paul Eckert, Louis Charbonneau and Arshad Mohammed
UNITED NATIONS, Sept 24 (Reuters) - U.S. Secretary of State John Kerry, in a move that puts the Obama administration at odds with the powerful American gun lobby, will sign the U.N. Arms Trade Treaty regulating the $70 billion international conventional arms business, diplomats said on Tuesday.
A senior State Department official said President Barack Obama's administration would notify the U.S. Senate on Tuesday and Kerry would sign the treaty on Wednesday on the sidelines of the annual U.N. General Assembly in New York.
Obama gave a speech to the assembly on Tuesday that focused on Syria, Iran and other Middle East hot spots.
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 United States 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.
President Goodluck Jonathan of Nigeria, whose country has been repeatedly attacked by a cross-border Islamic jihadist militant group called Boko Haram, told the United Nations such rebellions are "sustained by unfettered access by non-state actors to illicit smart arms and light weapons."
"For us in Africa these are the weapons of mass destruction," he said.
The U.N. General Assembly adopted the treaty on April 2 by a vote of 154 for, including the United States, three against, and 23 abstentions. The no votes were cast by Iran, North Korea and Syria, U.N. records showed.
WAY TO PRESSURE RUSSIA ON SYRIA
The NRA, which has opposed the treaty from the start, called the April vote a sad day for the United States, the world's No. 1 arms exporter.
Among the NRA's arguments against the treaty are that it undermines American sovereignty and disregards the Second Amendment of the U.S. Constitution, which guarantees citizens the right to bear arms.
The senior State Department official rejected the NRA's characterization of the treaty, saying the pact's target is "illicit trade in conventional weapons that benefits terrorists and rogue agents."
"The treaty recognizes and protects the freedom of both individuals and states to obtain, possess and use arms for legitimate purposes," said the official.
"It merely helps other countries create and enforce the kind of strict national export controls the United States has had in place for decades, which haven't diminished one iota the ability of Americans to enjoy their rights under our Constitution."
The U.N. Office for Disarmament Affairs has said the treaty would not "interfere with the domestic arms trade and the way a country regulates civilian possession."
"It will prevent human rights abusers and violators of the law of war from being supplied with arms. And it will help keep warlords, pirates, and gangs from acquiring these deadly tools," the U.N. office said on its website.
Frank Januzzi, deputy executive director of Amnesty International USA, called the move "a very significant win for 20 years of human rights activism" by his organization and by Oxfam International, a confederation of groups focused on poverty and injustice.
Januzzi said the treaty could be applied to the conflict in Syria, making arm sales to the government illegal under international law. Russia, Syria's main arms supplier, and China abstained in the April U.N. vote and have not signed the pact.
"This will increase the pressure on Russia to sign. It will increase the pressure on China as well," he said in a telephone interview. (Additional reporting by Michelle Nichols.; Editing by Xavier Briand and Christopher Wilson)