BEIJING (AP) — China's product safety agency has told New Zealand suppliers of a milk product to perform additional quality tests following a new contamination problem.

The agency said Tuesday it has impounded 390 kilograms (860 pounds) of lactoferrin powder supplied by Westland Milk Products Co. after the company said it found high levels of nitrate in the food supplement.

Westland said cleaning products containing nitrates were not properly flushed from its factory.

The General Administration of Quality Supervision, Inspection and Quarantine said all New Zealand suppliers of lactoferrin must test it for nitrate and Westland must test all its products.

New Zealand milk has faced heightened scrutiny in China after supplier Fonterra found bacteria that can cause botulism in its baby formula ingredients.