UN leader Ban Ki-moon led international appeals Monday for major powers to make concessions to secure 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 said at the opening of negotiations, highlighting that "there are common standards in the trade of armchairs 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 countries' 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 could be used in human rights violations, terrorism or organized crime.

An attempt to finalize an Arms Trade Treaty in July last year failed after various countries put up obstacles.

A compromise document was drawn up but the United States followed by Russia and others, said they needed more time to study the accord.

The United States 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.

The conference started as the Stockholm International Peace Research Institute said that China had overtaken Britain to become the fifth largest arms exporter.

Some 108 countries released a statement at the meeting calling for "considerable improvement" in the negotiating text. Most European nations signed the accord, but the United States, Russia, China did not.

And diplomats at the talks are very cautious about the chance of reaching an accord.

"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.

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. This he added, would be like "reopening Pandora's Box."

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

Tuomioja called ammunition "one of the core issues". He said the draft accord with ammunition left in an annex, which would mean its trade was not monitored, was "inadequate."

The world ammunition market is estimated at $4 billion dollars a year with the United States accounting for half of that, and the US government is under pressure from the gun lobby to oppose an accord.

US Secretary of State John Kerry said on Friday that the United States wants a "strong and effective" treaty.

But his State Department reaffirmed it opposes the inclusion of ammunition because of the financial and administrative burden of keeping checks.

Oxfam and Amnesty International have also 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 help for Syria's President Bashar al-Assad.

The United States helps Egypt and France its African allies in this way.