Disarmament vows have failed to reduce the nuclear threat as nations invest in new weapons systems, a leading think-tank said Tuesday, amid a rise in global tensions due to competition for scarce resources.

"More than 5,000 nuclear weapons are deployed and ready for use, including nearly 2,000 that are kept in a high state of alert," the Stockholm International Peace Research Institute (SIPRI) warned in its annual report.

The world's eight nuclear powers -- Britain, China, France, India, Israel, Pakistan, Russia and the US -- possess more than 20,500 warheads, the think-tank said.

While this was a reduction of more than 2,000 warheads compared to 2009, SIPRI stressed the drop was largely balanced by new and planned investments in nuclear arms technologies, arguing that prospects for meaningful disarmament in the short term were grim.

"The nuclear weapons states are modernising and are investing in their nuclear weapon establishments, so it seems unlikely that there will be any real nuclear weapon disarmament within the foreseeable future," SIPRI Deputy Director Daniel Nord told AFP.

All five legally recognised nuclear weapons states, Britain, China, France, Russia and the United States, were already deploying new nuclear weapon systems or had announced they would soon do so, he pointed out.

As of January 2011, Russia had 11,000 nuclear warheads, including 2,427 deployed, while the United States had 8,500 including 2,150 deployed, the report said.

Both countries have signed a Strategic Arms Reduction Treaty (START) that went into effect in February and calls for a maximum of 1,550 warheads deployed per country.

When asked what constituted the biggest nuclear threat today, Nord pointed to India and Pakistan, which are expanding their nuclear arsenal.

South Asia, Nord said, was "the only place in the world where you have a nuclear weapons arms race."

While also mentioning the dangerous situation in North Korea, which "is believed to have produced enough plutonium to build a small number of nuclear warheads," according to SIPRI, Nord said Iran for now was not on his list of countries posing a significant nuclear threat.

The risk is not that Iran will use nuclear weapons, he said, but rather "what will be the consequences when the concerned states like Israel or the United States decide that they will have to intervene and do something about the programme in Iran."

SIPRI's annual report also said growing competition for scarce or valuable natural resources is causing or fuelling numerous conflicts around the world.

Neil Melvin, the head of SIPRI's Armed Conflict and Conflict Management Programme, said it is difficult to pinpoint direct conflicts between countries over resources, but "there is certainly growing tensions... contributing to conflicts breaking out... and especially driving existing conflicts."

"Look today at the Democratic Republic of Congo, where violence around mining activities there is a leading factor in the violence in that country," he told AFP, pointing out that there is a global link to this conflict, since "everyone's mobile phone probably has minerals from Congo in them."

Water scarcity and higher food prices were also sparking conflict, he said.

"Now there are projections that food prices may double up to 2020, driven by scarcity of food but also climate change," Melvin said.

"We have seen examples of how this can be linked to violence with the Arab Spring, in which the onset of violence in Tunisia, and to some degree also in Egypt, was sparked by food rioting and individuals responding to high food prices," he said.

The SIPRI think-tank, which specialises in research on conflicts, weapons, arms control and disarmament, was created in 1966 and is 50-percent financed by the Swedish state.