Airmail service between Chicago and Washington was disrupted yesterday as three airlines declined for a while to carry the mail after Thursdy's explosion in a mail pouch forced a smoke-filled American Airlines jet to land at Dulles International Airport.

FBI bomb specialists labeled the bomb "an incendiary device" that was designed more to burn than to blast. However, FBI officials said, much of the brown wrapping paper -- including writing indicating both destination and return address -- survived the fire in the jetliner's cargo compartment.

In Chicago, where American Flight 444 originated Wednesday, one newspaper and two radio stations received calls claiming that the bomb had been mailed by the Iranian Student Association. "There will be more bombs, larger bombs, unless the deportation of Iranian students is stopped," a caller told a deskman at the Chicago Tribune.

The calls in Chicago came 12 hours after the incident aboard the American flight. The FBI said it took the calls seriously and was investigating them, but a spokesman said it was unusual for a claim of responsibility to come so many hours after the occurence. "Usually such calls come immediately before or immediately after," the spokesman said.

In Washington, a spokesman for the Iranian Student Association denied responisbility. "There is no way the ISA or any other Iranian student group could have done it. It's a lie," the spokesman said. The Iranian Student Association has many factions, according to those familiar with it.

Meanwhile, American, United and Trans World airlines -- the three major nonstop carriers between Chicago and Washington -- embargoed mail from their planes for a while yesterday. By evening, however, all three airlines were carrying letter pouches, but packages were being moved by other means, according to spokesmen for the airlines.

The incendiary device in the American plane was contained in a wooden box described by the FBI as being 9-1/2 inches by 10-1/2 inches by 7-1/2 inches. The box contained the device, three size D dry cell batteries and an altimeter triggering device. An altimeter measures altitude and was apparently set to ignite the device at "a high altitude," the FBI said.

The package was mailed in the Chicago area, according to Charles Monroe, special agent in charge of the Fbi's Alexandria field office. Stamps totaling $8.50 were used to pay the postage.

Top officals of the Federal Aviation Administration, which is responsible for airport security enforcement, met with U.S. Postal Service officials yestersday afternoon to discuss ways of improving security of mail pouches.

Mail pouches are not normally screened, FAA officials said. That is because, they said, it is impossible to target a specific flight with a mailed letter because mail is dispatched on a first-plane-out basis.

Jamison Cain, a spokesman for the Postal Service, said that yesterday's short-lived embargo on mail flights should have no effect on letters. "We are discussing with the airlines the shipment of priority packages," Cain said, "and in the meantime are seeking alternate transportion for these packages." He said there should be "no significant delays."

Passengers aboard Flight 444 told The Washington Post Wednesday that they heard a "thump" shortly after the plane left O'Hare International Airport in Chicago about midday. The jet continued to Washington, but smoke began to fill the cabin and the pilot landed the plane at Dulles instead of Washington National because it was closer.

American Airlines spokesman Al Becker, asked why the pilot had waited through most of the flight to Washington before landing, gave this explanation:

About 20 minutes after takeoff from Chicago, flight attendants heard a thud and smelled an odor. The flight engineer (the No. 3 man in the cockpit) was summoned to take a look.He did, but the odor was gone and there was nothing to see. There was no indication in the cockpit that anything was amiss.

About 30 minutes before a scheduled 12:57 p.m. landing at National Airport, smoke began to fill the cabin. Passengers were herded to the rear and the oxygen masks dropped. The captain then changed his destination to the somewhat closer Dulles field.

None of the 80 people on board was injured. They all left by the rear exit of the plane. Damage to the plane was minor, officials said.

The FAA and the National Transportation Safety Board also are conducting inquiries into the incident.