The Transportation Security Administration said yesterday that it will delay issuing new security screening rules for large charter airplanes because the task is more complex than it originally thought.

Security experts say charter flights remain vulnerable to terrorists because charter companies -- even those that carry groups of vacationers and sports teams on large jets -- are not required to screen passengers or luggage. The TSA originally imposed a Dec. 1 deadline on about half a dozen operators of large charter planes -- those weighing more than 95,000 pounds -- to begin new screening methods, but it had not told the companies what additional steps to take. Yesterday, a TSA official told the charter industry's largest association, the National Air Transportation Association, that it would issue security rules "shortly" and that charter companies would have 30 days to comply.

"We thought we'd get it out by December 1, but we're not going to make December 1," said Heather Rosenker, a TSA spokeswoman. Rosenker said the agency would come up with new rules "within weeks" because "it's an area of vulnerability."

Jim Coyne, president of the charter association, said it's difficult to come up with uniform security rules for the charter industry because its offerings include private services, such as flying corporate executives, as well as services for the public, such as tour operators that fly seasonally to vacation spots.

While the TSA has focused on addressing security at the 429 airports that have scheduled commercial flights, charter operators use as many as 5,000 smaller airports, many of which are not equipped with metal detectors and explosive-detection machines used in major airports with commercial airlines.

"TSA has not really had the resources internally to deal with the amount of effort that was required" with security rules for charter flights, Coyne said. "I think they underestimated."