Problems with security at the Athens airport were reported more than three years ago, but negotiations with the Greek government have not improved passenger security screening, according to an official of the International Aviation Transport Association.

As a result, the official said, TWA set up a second screening post in the airport so passengers were screened twice before they boarded TWA flights.

Despite the precaution, hijackers armed with what they said were hand grenades seized an Athens-to-Rome TWA flight early yesterday with 153 people aboard. Most of the passengers and all of the crew are Americans.

Rodney Wallis, director of Facilitation and Security for the International Aviation Transport Association (IATA), said in a telephone interview that his organization had found problems with the motivation of the police force guarding the access to planes at the Athens airport. Also, guards were not kept abreast of information pertaining to their jobs, he said. He would not elaborate.

According to Wallis, IATA, whose members are international air carriers, sets minimum safety standards for international airports. IATA specifies that airports establish "sterile areas" where boarding passengers are screened for weapons and explosives before they are allowed to board.

A TWA spokesman in New York declined to comment on security measures used in Athens, but Wallis said the second screening area set up by TWA was policed by airline employes.

Wallis refused to give further details on security problems at the Athens airport or at other international airports that fail to meet the organization's standards. He said an open discussion of airport weaknesses could cause a spate of "copycat" hijackings.

One of the problems at the Athens airport is the transit lounge areas where passengers wait to transfer to other flights, according to an industry source. Those areas have not been closely monitored and problems have occurred there in the past, he said without elaborating.

An administration official said the State Department had once considered issuing a travel advisory for Americans using the Athens airport. The official said Athens had been considered a "weak link" for a long time.

Wallis said in the past, hijackers have managed to get weapons aboard airplanes via airline food and cleaning-service workers. The vulnerability of aircraft on the ground is an area to which the industry is going to have to devote more attention, Wallis said.