Greek and Egyptian aviation officials accused each other today of lax security arrangements that were said to have led to two highly publicized and violent airplane hijackings during the past five months.

The sudden upsurge of suspicion between the two countries reflected a common element in the hijacking of a TWA plane last June and of an Egyptair Boeing 737 last weekend. In each case, the plane was commandeered shortly after it left Athens -- but its point of origin was Cairo.

In Geneva, the International Air Transport Association (IATA), which includes most of the world's scheduled airlines, said it planned to send security experts to Cairo and Athens during the next week to investigate how the hijacking occurred.

A spokesman for IATA, David Kyd, said in a statement that the organization was "increasingly wondering if Cairo is being used to put arms on aircraft." He did not indicate what evidence IATA had to support this suspicion.

Greece and Egypt have a clear interest in absolving themselves of responsibility for the incidents, both of which resulted in the deaths of passengers and heightened concern about airport security.

An official Greek statement today implicitly criticized the Maltese government's action in allowing Egyptian commandos to storm the plane, saying Greece was not informed of the operation in advance. Of 17 Greek passengers on the plane, only five survived.

After the June hijacking, the Socialist government complained of losing millions of dollars of tourist revenue when the Reagan administration advised U.S. citizens against traveling through Athens airport. Today, Foreign Minister Ioannis Kapsis categorically ruled out a security lapse at the airport Saturday night.

Only sparse details have emerged so far about the background to the takeover of the Egyptian plane, which occurred over the Greek island of Minos after takeoff from Athens for Cairo.

The hijackers are believed to have boarded the plane here after spending at least two days in the Greek capital but, according to Greek and Egyptian sources here, it seems likely that their arms were smuggled aboard separately.

According to one hypothesis, by Greek airport officials today, the arms were already on the plane when it arrived from Cairo. The same theory was advanced after the hijacking by Shiite extremists of the TWA plane while it was on a flight from Cairo to Rome via Athens.

A second hypothesis, by Egyptian airline officials here, is that the hijackers were helped by accomplices at Athens airport. The Egyptian officials conceded today that it was difficult to imagine how the hijackers had managed to smuggle their guns and hand grenades through three different airport security checks, including two operated by Egyptair rather than Greek authorities, without outside help.

Security at Athens airport has been tightened since the TWA hijacking and it was declared to have reached acceptable international standards by IATA.

Kapsis said today that the security lapse must have taken place "somewhere else." Although he refrained from naming the Cairo airport, Greek officials who asked not to be named said this was the only alternative.

The idea that the arms already could have been on the Boeing 737 was dismissed by the Athens station manager for Egyptair, Kamal Fakharani, who described Cairo airport as one of the world's safest.

"We had four security men on that plane. They searched the whole plane thoroughly both before taking off from Cairo and when it arrived in Athens," he said.

Initial attempts at reconstructing the movements of the hijackers who took over the Egyptair plane compound the mystery -- while shedding some light on how the operation was planned. The suspect passenger tickets, which Fakharani displayed to this reporter, show that at least three of the estimated five hijackers made their final flight arrangements in Athens last Thursday.

The tickets indicate that two of the hijackers flew to Athens from Belgrade, which has long been considered a convenient transit point for international terrorists because of Yugoslavia's "open border" policy and excellent relations with most Arab states. The Palestinian guerrilla leader accused by the United States of masterminding the Achille Lauro hijacking, Mohammed Abbas, spent several days in Yugoslavia after being allowed to leave Italy.

One of the suspect flight tickets, a Yugoslav Airlines ticket in the name of S. Bou, was bought in Belgrade on Nov. 15 with Athens and Cairo as the destination points. A second Athens-Cairo-Addis Ababa coupon, made out in the name of Chakorf, had been issued in exchange for a ticket originating in Belgrade on Nov. 18.

The third coupon, issued in the name of Marzouki, was part of a simple Athens-Cairo return ticket issued in Athens on Thursday.

According to Egyptair officials, normal security arrangements were in force at Athens airport Saturday night when the plane was hijacked. These involved an initial search of luggage by a Greek security firm hired by Egyptair at check-in, a search of passengers by Greek government staff on their entering the transit lounge, and a second metal-detector search by Egyptair staff on their leaving the transit lounge.

With the exception of the initial physical search, similar controls were in force for the 27 transit passengers who caught the Cairo-bound Egyptair flight Saturday. Fakharani said, however, that the hijackers did not appear to have been among the transit passengers.

Other acknowledged weak points at Athens airport include the 11-mile perimeter fence, which lies close to a built-up area. Police in armored cars are patrolling the somewhat flimsy wire fence until it can be replaced by an electronically guarded enclosure, to be reinforced by eight special towers.

In line with normal practice, Saturday's Cairo-bound Boeing 737 was cleaned by four Greek staff who have worked for Egyptair for a number of years. Catering supplies were provided in Cairo, Fakharani said.

Asked if he could take any further steps to improve security, Fakharani replied: "Perhaps I will now check bodies and handbags at the steps of the aircraft. It is always theoretically possible for machines to make mistakes."

Although the equipment used in Athens is standard for international airports, there has been criticism of the poor training of, and long hours worked by, Greek security officials.

"They tend to get distracted from the machines. If it keeps going beep, the natural inclination is to assume that the machine must be faulty," said Steve Bassi, the director of a British travel firm that sends thousands of British tourists to Greece every year.