Chinese officials knew for weeks about sales of chemical-tainted milk powder that has killed two infants and sickened 1,253, but did not act until the New Zealand government pressed Beijing, New Zealand's prime minister said yesterday.

The sale of tainted infant formula by Sanlu Group, the Chinese producer 43 per cent owned by Fonterra Dairy Co-operative of New Zealand, was reported for the first time by state media last week, sparking panic and outrage among parents.