The quarantine administration said Thursday that its Shenzhen bureau has this year detected harmful pine wood nematodes (microscopic worms) in 13 batches of wooden packaging containing imported goods from the United States.

The packaging was produced by six manufacturers with official US certificates, the Administration of Quality Supervision, Inspection and Quarantine (AQSIQ), said.

The city's quality supervision official also discovered harmful living organisms in a further 10 batches of wooden packaging from the US, out of 70 batches sampled between July 14 and August 23, the administration said.

The proportion of substandard wooden packaging from the US - 15.38 percent of the sample - was much higher than that for the European Union, Republic of Korea, Japan or Canada, the AQSIQ said.

The administration believed that either the manufacturers or exporters were to blame for the problems with the packaging.

It said the labels on some packaging were not clear, suggesting some exporters might have used fake ones.

The administration said the seized packages had been improperly treated according to quarantine requirements, and it urged local officials to tighten the supervision of wooden packaging from the US.

A day earlier, the administration said a cargo of 21.6 tons of frozen potato slices imported from the US was not up to standard in terms of carbonyl and acidity levels.

The goods were destroyed according to relevant laws and regulations. The administration notified the US side and asked it to investigate the matter.