Chinese companies banned from doing business in the U.S. for allegedly selling missile technology to Iran continue to do a brisk trade with American companies, according to an analysis of shipping records.

A unit of state-owned China Precision Machinery Import-Export Corp., for example, has made nearly 300 illegal shipments to U.S. firms since a ban was imposed on CPMIEC and its affiliates in mid-2006, according to an analysis of shipping records by the Wisconsin Project on Nuclear Arms Control, a nonprofit proliferation watchdog.

A Wall Street Journal review of the records and interviews with officials at some of the American companies indicate that the U.S. firms likely were unaware they were doing business with banned entities, and in many cases were tripped up by altered company names.

The CPMIEC shipments, worth millions of dollars, include everything from anchors and drilling equipment to automobile parts and toys. In many cases, CPMIEC acted as a shipping intermediary -- activity also banned under a 2006 presidential order.

The ability of CPMIEC and other foreign companies to continue doing business in the U.S. despite the sanctions comes as the Obama administration considers fresh economic sanctions against Iran. The illegal shipments suggest that U.S. sanctions have become so numerous and complex that they have become difficult to enforce.