US President Barack Obama in 2010 hosted a first summit in Washington, aimed at securing or destroying the world's stocks of plutonium and highly enriched uranium (HEU) by 2014.

Despite some major achievements since then, however, a tepid final communique released on Tuesday at the end of the second summit in Seoul indicated an uncertain road ahead for the process, experts said.

"What we really needed from this summit was a little bit more vision on how to move beyond where we are today," said Ken Luongo, co-chair of the Fissile Materials Working Group (FMWG), a grouping of non-proliferation experts.

Luongo said the blueprint going forward had been watered down because of the varying commitment levels of countries around the world.

"The problem with this process is that it has devolved to a lowest common denominator through the negotiation process," he said.

"The challenge of this summit process going into Netherlands (the next summit in 2014) is to open up the vision so that there is a big picture perspective on what we need to do to prevent nuclear terrorism."

The communique, representing the views of all 53 nations at the summit, pledged strong action on safeguarding the world's stockpiles of nuclear material.

It emphasised the need to secure stocks of highly enriched uranium (HEU) and plutonium, small amounts of which can be used to make devastating bombs.

Duyeon Kim, a deputy director at the Center for Arms Control and Nonproliferation, said setting the target of the end of 2013 to minimise the civilian use of HEU, such as in the medical sector, was a "positive step".

"But so far it's an 'encouragement' to do so and the key is in the details, which are unclear," Kim said.

Experts highlighted much progress since the Washington summit, such as removing enough HEU to make 19 nuclear weapons from eight countries.

But experts described the measures taken since 2010 as low hanging fruit, and that momentum would be difficult to sustain without binding and transparent agreements among all countries.

"The current nuclear material security regime is a patchwork of unaccountable voluntary arrangements that are inconsistent across borders," Luongo said.

"Consistent standards, transparency to promote international confidence, and national accountability are additions to the regime that are urgently needed."

"Nuclear terrorism continues to be one of the most challenging threats to international security," said a joint communique at the end of a two-day summit in South Korea bringing together leaders or top officials from 53 nations.

"Defeating this threat requires strong national measures and international cooperation."

The leaders, including US President Barack Obama and Chinese President Hu Jintao, stressed the "fundamental responsibility" of all nations to safeguard nuclear material and keep it out of the hands of terrorists.

It also urged all countries to accede to international conventions on protecting fissile material, and reaffirmed the central role of the UN's atomic watchdog, the International Atomic Energy Agency.

One of the key points in the communique was an emphasis on the need to secure stocks of HEU, which is used to make weapons but also in nuclear power plants and medical devices.

The communique called for nations to minimise the use of HEU, with nations encouraged to announce by the end of 2013 how they would do so.

This could include the conversion of reactors from HEU to low-enriched uranium fuel which cannot be weaponised.

The communique called for effective inventories and tracking mechanisms for nuclear material and the development of forensics capacities to determine its source.

The leaders also welcomed "substantive progress" on national commitments made at the first nuclear security summit in Washington in 2010.

This included the disposal of 480 kilogrammes (1,060 pounds) of HEU -- enough to make about 19 nuclear weapons -- from eight countries.

Ukraine and Mexico have cleaned out all stockpiles of HEU, while Russia and the United States have converted HEU equivalent to 3,000 nuclear weapons down to low-enriched uranium.

Experts said modest progress had been made in Seoul but cautioned many of the tough issues to fully solve the problem had not been addressed, with countries unwilling to make binding and transparent agreements.

"The current nuclear material security regime is a patchwork of unaccountable voluntary arrangements that are inconsistent across borders," said Ken Luongo, co-chair of the Fissile Materials Working Group, a group of non-proliferation experts.

"Consistent standards, transparency to promote international confidence, and national accountability are additions to the regime that are urgently needed."

The final communique also omitted a reference to the need for "concrete steps" towards a world without nuclear weapons, a phrase which had been included in a draft statement dated March 21 and seen by AFP.

The leaders said merely that they "reaffirm our shared goals of nuclear disarmament, nuclear proliferation and peaceful uses of nuclear energy".

A Seoul government official told AFP on condition of anonymity that some nations had been uncomfortable about expanding the scope of the summit into nuclear weapons reduction and disarmament, and the call for concrete steps.