Washington is adamant that six-party talks remain the best way to disarm North Korea, in spite of Pyongyang's increasing antagonism and “convincing” evidence that the country has exported weapons-grade uranium to Libya, according to the new US assistant secretary of state for east Asian affairs.

Although the fourth round of talks is seven months overdue, Christopher Hill said Washington's patience had not yet worn thin. “I'm not quite prepared to pull the plug [on the talks process],” said Mr Hill, the former ambassador to Seoul who starts his new job in Washington on Tuesday. “I still think it is the best mechanism for dealing with this and I hope the North Koreans will come round.” Asked when the stand-off will have reached the point of no return, Mr Hill declined to set a deadline, saying: “We'll know it when we see it but we're not there yet.”