A user on a Web site that carries al Qaeda rhetoric has posted a threat to attack the under-construction 1 WTC tower with more than a dozen aircraft, sources said yesterday.

The Shamikh Tier I Forum Thread was uploaded on Sept. 15 under the ominous heading “A new tower and a new operation.”

It showed an illustration of kamikaze airplanes flying toward a completed version of the skyscraper amid several smaller skyscrapers under a blue sky.

The NYPD and federal authorities are keeping a close eye on the site.