WASHINGTON (AP) _ U.S. Customs agents knew about illegal shipments of American-made helicopters to North Korea for 16 months without doing anything about it, a newspaper reported today.

According to an internal Commerce Department analysis, Customs officials received detailed intelligence information in September 1983, after the first two of 87 Hughes helicopters were diverted to North Korea, but took no action, The Washington Post said.

The document said the Commerce Department stepped in to halt the shipments early this month after learning of the scheme independently.

Theodore Wu, deputy assistant secretary of commerce for export enforcement, said on Feb. 3 that the shipments of Hughes Model 300 and Model 500 helicopters was ''probably the largest illegal diversion of U.S.-manufactured aircraft'' known to have occurred.

The helicopters would be enough ''to give close air-to-ground support for a whole regiment of troops,'' Wu said.

His comments came after Customs and Commerce Department agents armed with search warrants seized documents from two Los Angeles-area freight companies that Wu said were allegedly involved in packing and forwarding some of the helicopters.

A Commerce Department spokesman, Malcolm Barr, said he had no information about the internal analysis cited in the Post story.

The newspaper said the document was prepared by Commerce export control officials in an effort to determine how a West German company, Delta-Avia Fluggerate, was able to carry out the diversion.

U.S. officials have said there was no evidence to indicate that the manufacturer of the aircraft, Hughes Helicopter Co., knew anything about the scheme.