UN leader Ban Ki-moon led international appeals Monday for the major powers to reach a compromise on a landmark conventional arms trade treaty as final negotiations started.

The 193 UN members have until March 28 to conclude an accord on the $80 billion a year trade in small arms, tanks, warships, combat aircraft, ammunition and missile launchers.

"The absence of the rule of law in the conventional arms trade defies explanation," Ban told the opening of the negotiating conference, highlighting that "there are common standards in the trade of arm chairs but not in arms."

Armed conflict kills more than 500,000 people a year and some Latin American drug cartels are now better armed than the national armies where they operate, the UN leader added.

"Now is the time for focus and political will to negotiate the final details of the treaty," Ban said.

The treaty would aim to force countries to evaluate, before making a sale, whether weapons will be used for human rights violations, terrorism or organized crime.

An attempt to finalize an Arms Trade Treaty (ATT) in July last year failed. A compromise document was drawn up, but the United States, followed by Russia and others, said they needed more time to study the accord.

Washington remains opposed to including ammunition in an accord. Other major arms producers such as Russia and China have also taken a tough stance, diplomats said.

"We do not know yet, at this stage, whether we will succeed," said Finland's Foreign Minister Erkki Tuomioja, whose country has been one of seven leading nations behind the conference.

At the opening of the talks, Tuomioja appealed to all states to "be prepared to make compromises and find the political will to agree on the arms trade treaty."

"The first red line must be to not weaken the treaty" drawn up last July, said France's chief negotiator, Jean-Hugues Simon-Michel, adding that doing so would amount to "reopening Pandora's Box."

The inclusion of ammunition remains a major obstacle to an accord.

Tuomioja said ammunition was "one of the core issues in the ATT and should be treated as such." He said the draft accord with ammunition relegated to an annex, and therefore essentially unmonitored, was "inadequate."

The US State Department reaffirmed Friday that it opposes any treaty that includes ammunition because of the financial and administrative burden of keeping checks.

The United States -- which is believed to account for half of the estimated $4 billion a year ammunition market -- is under pressure from the influential National Rifle Association and other groups to oppose an accord.

"The United States is steadfast in its commitment to achieve a strong and effective Arms Trade Treaty," said Secretary of State John Kerry.

But he added that his country, the world's top arms producer, could only agree on a "treaty that addresses international transfers of conventional arms solely."

Oxfam and Amnesty International have meanwhile condemned the absence of spare parts from the draft text. Oxfam estimates this market at $9.7 billion dollars between 2008 and 2011.

The text also does not cover transfers of arms carried out as part of military cooperation accords, such as Russia's military aid to Syria's President Bashar al-Assad.

The United States helps Egypt and France assists its African allies through similar accords.