"If they want to get isolated, we'll be happy to isolate them," he said without elaborating.

The communist state refused to rejoin negotiations on its nuclear program on the sidelines of the ASEAN Regional forum on security issues in Malaysia last week.

This prompted its previous nuclear negotiating partners South Korea, China, Japan, the United States and Russia -- along with Australia, Canada, Indonesia, Malaysia and New Zealand -- to hold talks without the regime.

Hill was speaking after a visit to the Philippine military academy in the northern resort city of Baguio.

In Kuala Lumpur on Friday, US Secretary of State Condoleezza Rice urged North Korea to rejoin negotiations on its nuclear programme, saying the United States was ready "at any time, at any place and without any conditions" to meet it under the six-nation talks framework that began three years ago.

But the North, which left the talks in November and caused outrage this month when it test-fired seven missiles, said it would not return until US financial sanctions against it were dropped.

It said it was also considering withdrawing from the ASEAN Regional Forum, one of the few diplomatic gatherings it attends.

Hill said he was optimistic North Korea could still be persuaded eventually "to think clearly and come back to the dialogue."

North Korea walked out of the talks in November after Washington accused a Macau-based bank of helping it launder earnings from fake US currency, and told US financial institutions to stop dealing with the bank.

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