The government has run an efficient checking system on pesticide residues, but it is difficult to trace the source of vegetables by the time they reach market, said Leung Wing-lup, director of the Food and Environmental Hygiene Department.

Leung's remarks came in response to a report by environmental groups that said vegetables sold in Hong Kong's supermarkets contain harmful levels of pesticide residues.

He said the government has required major supermarkets to set up a better system for tracking the source of veggies.

The Food and Environmental Department conducted spot checks on 20,000 vegetable samples last year, one-fourth of which were taken from supermarket shelves.

Hong Kong imports 1,700 tones of vegetables a day.