Tony Snow, the White House spokesman, said: "The United States is not going to engage in bilateral negotiations with the government of North Korea."

He said that Washington was sticking to its position that any negotiations be conducted through a six-nation format.

North Korea's official news agency, KCNA, had quoted as foreign minister as saying on Thursday: "If the US has a true political intention to implement the joint statement we kindly invite once again the head of the US side's delegation to the talks to visit Pyong Yang and directly explain it to us."

Christopher Hill, the chief US envoy to the talks also involving South Korea, China, Japan and Russia, has previously said he would visit Pyongyang.

As part of last September's joint statement, North Korea agreed to halt its nuclear weapons programme in September in return for aid and security assurances from the US.

But North Korea refused to return to the negotiations in protest at the US blacklisting firms allegedly aiding Pyongyang in illegal financial activities.

"The US escalated hostile policy and increasing pressure upon the DPRK [North Korea] would only compel it to take the strongest measures to protect its right to existence and sovereignty," the statement said.

North Korea has sought bilateral talks with the US for some time.