WELLINGTON (REUTERS) - New Zealand police said on Tuesday that a threat had been made to contaminate infant formula as a protest against the use of an agricultural pesticide.

A trading halt was placed on shares in Fonterra, the world's largest dairy exporter. Shares in smaller dairy firms A2 Milk and Synlait Milk were also suspended.

Anonymous letters had been sent to a national farmers' group accompanied by contaminated packages of infant formula, with a threat of contamination unless use of the poison known as "1080" ceased by the end of March.

The Ministry of Primary Industries said infant and other formula was safe, and police said supply chain security had been increased.

Police said they were treating the issue as blackmail.