New Zealand says it is unlikely to act over claims the brother of a French presidential candidate was behind the 1985 bombing of a Greenpeace ship.
Segolene Royal said she was "surprised" at the controversy
Segolene Royal's brother Gerard planted the bombs that sank the Rainbow Warrior in Auckland harbour, another brother, Antoine Royal, said at the weekend.
The New Zealand government said the case had been closed since 1991 and was unlikely to be re-opened.
The attack on the ship killed photographer Fernando Pereira.
French secret service agents planted two bombs on the sides of the ship in an attempt to sabotage Greenpeace's campaign against France's nuclear testing in the Pacific.
Two agents - Alain Mafart and Dominique Prieur - were arrested after the incident. They were sentenced to 10 years in jail after pleading guilty to manslaughter but were released into French custody a year later.
A French suspect was arrested in Switzerland in 1991, but New Zealand dropped extradition proceedings and stayed all outstanding charges over the bombing on the grounds of national interest.
New Zealand Prime Minister Helen Clark said on Monday: "Were New Zealand now to endeavour to re-open the case, it is likely the French government would consider we were acting contrary to earlier undertakings.
"The advice I have from the Ministry of Foreign Affairs and Trade is that France was held to account for its actions at international law," she told reporters.
The attack led to tensions between New Zealand and France
Antoine Royal was quoted by France's Le Parisien newspaper as saying that his brother Gerard had been a lieutenant and agent of the French foreign intelligence agency, DGSE, in Asia at the time.
"He was asked in 1985 to go to New Zealand, to Auckland Harbour, to sabotage the Rainbow Warrior," he is reported to have said.
His comments came as Segolene Royal officially declared her candidacy for the nomination of France's opposition Socialist Party, due to be voted on in November.
Opinion polls make 53-year-old Segolene Royal the strong favourite to be the party's candidate fighting next April's presidential elections.
She said she was "a bit surprised at all this controversy springing up just after I declared my candidacy. I don't know if it's a coincidence."
She said that if there was anything new to be said on the issue, the defence ministry should say it and praised her brother Gerard as a "great soldier".
Executive director of Greenpeace in New Zealand, Bunny McDiarmid, said it would be a "fruitless exercise" re-opening the case.
"There is very little they can do. I think the New Zealand police which put an enormous effort into tracking down at least two of the agents... probably feel as disappointed and frustrated as Greenpeace did," she told New Zealand television.