The US Treasury Secretary, Timothy Geithner, who appeared with Mrs Clinton at the news conference, warned that international banks should be wary of doing any business with Iran that might aid its nuclear programme.

BBC: Iran faces fresh Western sanctions over nuclear plans

Russia has also been trying to persuade the North to restart regional talks over its nuclear-weapons programme in return for more aid.

ECONOMIST: A hermit makes a rare venture out of his kingdom

After the talks in Beijing ended, it offered to provide North Korea with aid for its bankrupt economy provided Pyongyang scrapped its nuclear-arms programme.

ECONOMIST: Dangerous and nuclear