The UN conference on fighting the spread of nuclear weapons opened last week with Iranian President Mahmoud Ahmadinejad blasting the United States, but now comes the hard part of bridging gaps among nations.

Ahmadinejad's charges that Washington is threatening nuclear attacks and Secretary of State Hillary Clinton's sharp dismissal of these accusations as "wild" raised fears of a stalemate at the conference on the 189-nation Non-Proliferation Treaty (NPT).

The nitty-gritty began Friday, with committee kicking off after four days of opening speeches. The meeting runs until May 28 at UN headquarters in New York.

The success of the NPT is that the number of nations with the atomic bomb is still less than 10. In 1970, when the treaty went into effect, there were fears there would soon be dozens of states with the bomb.

But the NPT is in crisis, with questions about how to monitor suspicious nuclear programs in Iran and North Korea and how to get nuclear-weapon states to carry out their treaty promise to move towards disarmament.

The previous NPT review failed in 2005 to reach agreement. There is concern the same issues that torpedoed that gathering, including divisions over Iran's nuclear ambitions, could sabotage this year's event.

But Susan Burk, the US special representative for nuclear non-proliferation, told AFP on Friday that "in 2005 the conference was bogged down for two weeks over procedural issues."

The NPT review is held every five years.

This year, the conference had by last Wednesday "cleared away the last procedural item," Burk said, referring to setting up committees related to the three pillars of the NPT -- disarmament, the peaceful use of nuclear energy and fighting proliferation by monitoring national nuclear programs.

She called this "a very positive development and a first indication that this would not be like 2005 at all."

US President Barack Obama has made a concerted effort to move from his predecessor George W. Bush's tactic of confrontation to a new policy of engagement.

A series of disarmament initiatives over the past year has won the United States credibility on the non-proliferation front.

"Where the United States was part of the problem and obstructing in 2005, countries here see the Obama administration as trying to deal with nuclear problems and threats," Rebecca Johnson, a leading non-proliferation activist, told AFP.

The coming weeks will show if this carries over to concrete progress in such matters as being able to impose penalties on states, like North Korea, which withdrew from the treaty.

Specific measures have little chance of being adopted here, diplomats said.

There is even doubt the conference will be able to agree on a final document, which must be by consensus.

A major problem is the mistrust between the nuclear haves and have-nots.

The bargain of the NPT is that nuclear-weapon states work to disarm while other states forswear the bomb and get access to peaceful nuclear energy.

Iran is not alone in saying the nuclear powers are not doing enough to disarm and are meanwhile trying to cut off access to peaceful nuclear technology.

Iran is under three rounds of UN Security Council sanctions to get it to stop making potential atom bomb material but says it only seeks to use nuclear power to generate electricity.

Another stumbling block here is Egypt's insistence, backed by non-aligned states, that there be a conference on creating a nuclear weapons-free zone in the Middle East.

This would include Israel, which is not a member of the NPT and is believed to have nuclear bombs.

Israel says there must be peace in the region before setting up such a zone.

The United States is leading world powers in seeking a compromise on this issue with Egypt.

The 1995 review conference had called for working towards a Middle East nuclear weapons-free zone.

A push by Egypt to have this promise honored was a major factor in the stalemate at the 2005 review.

US officials have already begun dealing with the possibility that this year's conference will not produce a consensus document.

Burk said the United States is looking for "broad agreement on the importance of the NPT to international and regional security."

This could mean settling for less than a consensus conclusion but saying there was a victory in re-affirming the NPT and agreeing on an agenda for future work, including in such forums as the UN nuclear watchdog in Vienna, US, Russian and other officials said.