WASHINGTON (REUTERS) - A 25-year-old United States (US) ban on any firearms that can escape discovery by a metal detector is scheduled to expire next month, just as law enforcement officials say they are closely watching the proliferation of plastic guns made with 3-D printers.

Attorney General Eric Holder said on Friday that if the 1988 Undetectable Firearms Act is allowed to end, plastic guns made with the fast-evolving 3-D printing technology could be smuggled more easily.

"This is a very worrisome threat to law enforcement and to people who fly every day. We can't have guns legally in circulation that are not detectable by metal detectors," Mr Holder said in a statement.

The law is due to expire on Dec 9, according to the Justice Department. Democrats in the Senate and House of Representatives introduced legislation months ago to renew the ban, but their proposals have not gotten votes.

New 3-D printers have been used to build working firearms, in addition to medical devices, furniture and an array of other objects.

Americans generally may make guns for personal use, according to the Bureau of Alcohol, Tobacco, Firearms and Explosives, but the 1988 law prohibits anyone from making a gun that, after removal of grips, stocks and magazines, is imperceptible by a metal detector.

Mr Holder said a renewed ban on undetectable guns "should enjoy broad, bipartisan support."

Democratic Senator Charles Schumer of New York said in a statement that lawmakers "are actively exploring all options to pass legislation that will eliminate the threat of completely undetectable weapons."

President Barack Obama pushed new gun control measures such as expanded background checks this year in response to the December 2012 shooting at an elementary school in Newtown, Connecticut, but the legislation failed in the Senate. The House did not take it up.