The New Zealand navy is engaged in a stand-off with two boats thought to be illegally fishing toothfish in the Southern Ocean.
Naval patrol vessel HMNZS Wellington had been monitoring the vessels which are sailing under the flag of Equatorial Guinea, the foreign minister said.
Overnight authorities in Guinea gave New Zealand permission to board them.
But crew members barred authorities from boarding the vessels.
Foreign Minister Murray McCully said the HMNZS Wellington tried to board the ships which was its "legitimate right", but the vessels refused to co-operate.
"Due to the conditions and the evasive tactics of the masters, it was not possible to safely board these vessels."
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