By Jonathan Marcus BBC defence correspondent

US President George W Bush's main point was that the existing international regime to curb the spread of nuclear weapons is inadequate for the task.

The extent of Pakistan's nuclear black market is unknown

He spoke of the need for far more robust export controls, better intelligence co-operation and more active attempts at counter-proliferation.

A good example is the stopping and searching at sea of a freighter carrying alleged centrifuge parts which were designed in Pakistan, but bound for Libya.

But experts say that the president left out as many measures as he put in.

Many argue that an aspiring nuclear state would most likely seek to get technology, expertise and even fissile material from an existing nuclear power where controls are weak.

Pakistan and Russia are the two most cited examples.

Abdul Qadeer Khan's elaborate black market operation seems to have been broken, but its full extent inside Pakistan has not been revealed, in part because the US badly needs this strategically-placed country as an ally.

Russia is also a source of concern, as its once dominant nuclear weapons industry has now largely collapsed.

Mr Bush praised programmes to help safeguard sites and materials, but he offered no new money for the so-called Nunn-Lugar programmes which many analysts say are already badly under-funded.

And the president failed to make clear his administration's view on a fissile material cut-off treaty that would impose an international ban on the production of uranium and plutonium for nuclear weapons.