US President Barack Obama's administration signed a UN treaty Wednesday to track exports of firearms and other conventional weapons, offering a major boost to the pact despite opposition at home.

The United States is the world's largest exporter of conventional arms, accounting for 30 percent of the $90 billion global industry. The landmark treaty aims to curb weapons shipments to extremists and conflict areas.

Secretary of State John Kerry, signing on behalf of the United States in an oversized book at the UN headquarters, called the treaty a "significant step" for global peace efforts.

"This is about keeping weapons out of the hands of terrorists and rogue actors," Kerry said after signing the Arms Trade Treaty in front of UN officials.

"This treaty strengthens our security and builds global security without undermining the legitimate international trade in conventional arms," he said.

A spokesperson for UN Secretary General Ban Ki-moon hailed the US signature as having "particular significance" and said the treaty would "contribute to efforts to reduce insecurity and suffering for people on all continents."

Kerry devoted much of his remarks to assuaging concerns inside the United States, where the weapons industry holds sizable political sway.

The Senate needs to ratify the treaty, which has quickly galvanized US conservatives who are deeply suspicious of both gun regulations and the United Nations.

"This treaty will not diminish anyone's freedom," Kerry said.

"In fact, the treaty recognizes the freedom of both individuals and states to obtain, possess and use arms for legitimate purposes," he said.

If the Senate refuses to ratify the treaty, the United States could face the same situation as under the Kyoto Protocol on climate change in which it helped draft the pact but remained a conspicuous holdout.

"There's no possible way that he'll have the votes," Republican Senator Jim Inhofe said of Kerry and the arms treaty.

Inhofe likened the signature to the trip by Kerry, then a senator, to the 2009 UN summit in Copenhagen where he vowed US action on climate change. Legislation ultimately died in the Senate.

"He's doing it just for theater," Inhofe told AFP in Washington.

The United States hesitated at finalizing the arms treaty as Obama faced re-election last year.

But the United States was part of efforts that sealed the treaty earlier this year, although critics say it watered it down to avoid any international enforcement.

A tough fight ahead

Only three nations -- Iran, North Korea and Syria -- voted against the treaty at the UN General Assembly on April 2.

But 23 others abstained, including major players China, India and Russia.

Russia, which along with Iran is the chief supporter of Syria's embattled President Bashar al-Assad, said that the treaty was too vague and could be used for political ends.

The treaty requires states to prohibit shipments of weapons that could be used in human rights violations including "attacks directed against civilian objects."

Italy, the world's eighth largest arms exporter, on Wednesday became the fifth UN member and the first European Union nation to vote to ratify the treaty. The treaty comes into force once 50 nations ratify it.

Advocates for the treaty said that they expected more than 100 countries to sign it by the end of Wednesday, which comes during the annual UN General Assembly summit.

Kathi Lynn Austin, executive director of the Conflict Awareness Project which aims to track illicit arms trafficking, hoped that the US signature would motivate Russia and China to join the treaty and develop stricter export regulations.

"The recent terrorist attack on a shopping mall in Nairobi is a terrible reminder that no citizen, tourist or visitor of a country is immune from the human tragedy that results from a worldwide, unregulated arms trade," she said.

"No nation should shirk its responsibility to rapidly sign, ratify and implement the Arms Trade Treaty," she said.

The National Rifle Association, the powerful US lobby against gun regulation, vowed to fight ratification.

The group opposed the treaty's call on all countries to maintain records for at least 10 years of the import or export of weapons.

"The Obama administration is once again demonstrating its contempt for our fundamental, individual right to keep and bear arms," said the group's Chris W. Cox, referring to the guarantee under the Second Amendment of the US Constitution.