WASHINGTON (AP) _ People who want to mail items weighing more than a pound will have to start showing up at the post office in person.

The U.S. Postal Service announced Friday that, starting Aug. 16, it will change procedures for handling parcels to increase the security of commercial airlines carrying mail.

Concerns about security have increased following the crash of TWA Flight 800, though that disaster has not yet been blamed on a bomb.

The postal moves were made in cooperation with the Federal Aviation Administration. Similar steps were taken in California when the Unibomber threatened to bomb an aircraft from that state.

``The changes are designed to enhance security measures and to protect the traveling public, postal employees and postal contractors who transport U.S. Mail,'' Postmaster General Marvin Runyon said Wednesday.

Parcels carrying stamps and weighing more than 16 ounces, domestic or international, will have to be brought to post offices for entry with retail clerks under the new rules

Packages found in collection boxes will be returned to the sender, unless the recipient is within the local one-day delivery area. Generally, that means the Postal Service will deliver local parcels dropped in boxes, but not those that might travel on aircraft.

Packages returned to senders will be accompanied by a note advising the sender to take it to a post office. Already applied postage may still be used when the parcel is remailed since it will not have been canceled.

The post office said the restrictions will not affect Express Mail.