CHINA has banned shellfish imports from the US west coast, cutting off a US$270 million Pacific northwest industry from its biggest export market, reports National Public Radio affiliate KUOW.

China said it imposed the ban after recent shipments of geoduck clams from northwest waters were found to have high levels of arsenic and a toxin that causes paralytic shellfish poisoning.

The restriction applies to clams, oysters and all other two-shelled bivalves from Washington, Oregon, Alaska and northern California. Canada was not mentioned in the ban even though British Columbia waters are much the same as those off Washington State to the south and Alaska to the north.

US officials think the contaminated clams were harvested in Washington or Alaska. Right now they're waiting to hear back from Chinese officials for more details that will help them identify the exact source.

State and federal agencies oversee inspection and certification to prevent the shipment of tainted shellfish. Jerry Borchert of the Washington Department of Health said he's never encountered such a ban based on Beijing's assertion that US safeguards failed to screen out tainted seafood.

"They've never done anything like that, where they would not allow shellfish from this entire area based on potentially two areas or maybe just one area. We don't really know yet," Mr Borchert said.

The biggest blow could fall to those who farm or harvest large geoduck clams concentrated in Puget Sound, where about five million pounds of wild geoduck are harvested each year.