Three more Aegis warships will be upgraded to take on the ballistic missile defense role, bringing the number of ships with the capability to 21, Pentagon and industry sources confirmed last week.

The ships — two cruisers and one destroyer — will all be from the Atlantic Fleet, Lisa Callahan, Lockheed Martin's vice president for Maritime Ballistic Missile Defense programs, said during a teleconference about Lockheed's BMD programs.

Chris Taylor, a spokesman for the Missile Defense Agency — which oversees all Pentagon BMD programs — would not confirm that three ships have been selected, but acknowledged that "MDA and the Navy are discussing how to do two to four additional ships as soon as possible."

The Navy would make no official comment on the situation. But one source said "two to four" more ships would receive the upgrade. Pending notifications to Congress, an announcement might come as soon as next week, the source said.

Previously, the Navy and Lockheed have upgraded three cruisers and 15 destroyers with the ability to track and intercept enemy ballistic missiles. The Aegis BMD upgrade costs approximately $10 million to $12 million per ship, and includes new Aegis software and minor hardware adjustments.

Of the 18 ships already in the program, all but two are assigned to the Pacific Fleet and based at San Diego, Calif., Pearl Harbor, Hawai'i, and Yokosuka, Japan. The two Atlantic ships operate from Norfolk, Va.

But while the initial Aegis BMD effort was aimed at North Korean ballistic missiles, the U.S. also is concerned about Iran's potential to strike targets in Europe.

A BMD communications system test was carried out earlier last summer when the Pearl Harbor-based destroyer Russell in the Mediterranean Sea and the San Diego-based destroyer Benfold in the Persian Gulf worked "with one another in detecting, tracking, sharing information and engaging a simulated ballistic missile by sharing data via a number of paths," according to information from the 6th Fleet in Italy.

The first ships upgraded with Aegis BMD — or Aegis version 3.6 — were capable of handling long-range surveillance and tracking of enemy missiles and shooting down those while they were in the exo-atmosphere, or upper reaches of the Earth's atmosphere.

The latest upgrade, 3.6.1, adds the ability to intercept targets in the terminal phase when they're heading back toward Earth.