NORFOLK, Va. - The U.S., in response to advances in missile threats from Iran and North Korea in recent years, has become more invested in Navy cruisers and destroyers that carry a high-tech radar system and dozens of missile interceptors.

As a result, the ballistic missile defence destroyers and cruisers that carry the high-tech Aegis system are a growing capability that is in hot demand by military commanders across the Middle East, Europe and the Pacific.

"They give the capability to the combatant commanders that allows them to position (the ships) where there's a need, and we feel they're a pretty good investment," said Adm. Bill Gortney, commander of U.S. Fleet Forces in Norfolk. Unlike other missile defence systems, he said the ships are "able to sail to where the crisis is."

Anthony Cordesman, a national security expert at the Center for Strategic and International Studies in Washington, said the ships provide a layered defence that can quickly provide warning and details of a missile launch in areas of the world where there may be only a limited time to mount a response. And they present an added complication and deterrent for an adversary such as North Korea or Iran that may be trying to target a nearby U.S. ally — such as South Korea or Israel.

At the same time, however, he said the missile defence ships with their constantly improving sensors, radars and missiles have become a critical yet costly weapon.

"This is going to be an evolving technology duel," Cordesman said. "We will see steadily better missiles, warheads and countermeasures, and that means that ships will have to be upgraded." But he added that those things are considerably easier to upgrade than the military's pricey aircraft.

Still, the increasing requirements for the ships also exact another toll on the already strained naval forces. Commanders are routinely forced to extend the ships' deployments, keeping sailors at sea for longer periods and shrinking their time at home.

The USS Stout, which is pierside at the Norfolk Naval Station, returned from its deployment to the Persian Gulf region in June 2011, and its crew is now preparing to go back out this summer. While most Navy cruisers and destroyers deploy for about 6 1/2 months, and then spend more than three years at home, the missile defence warships are spending up to 7 1/2 months deployed and get a bit more than two years at home between tours.

"They are the most stressed, they have the highest operational tempo of all our forces," Gortney said. "What we're trying to do in the Navy is to meet that demand at an acceptable personnel tempo for our sailors and their families, as well as allow us to continue to do the maintenance so these ships go to their service life."

During a reporter's visit recently to the USS Stout, crew members were preparing for the eventual deployment. Loaded below the sun-streaked deck were dozens of Tomahawk missiles, while anchored above were large, sophisticated sensors and radars.

Below, in the operations centre, a ring of computer stations lined the room, combined into groups that could detect or follow a launch and those that communicate with others and co-ordinate the response strike. Overhead, displays lighted up with high-tech graphics and lines of trajectory.

Navy officials say that at this point they simply don't have enough of the ballistic missile defence ships to meet the demands while still adhering to the regular deployment schedules. So Navy leaders waived the rules governing lengths of deployments and time at home in order to keep the ships at sea longer, while warning of the potential consequences.

The rigours of the more frequent deployments will hasten the wear and tear on the ships, increasing their need for maintenance and repairs and potentially shortening their useable lifespans. And the more frequent pace of operations can also strain the crews, hurting efforts to recruit and retain quality sailors.

In all, the Navy has 28 ballistic missile defence ships — 16 are based in the Pacific and 12 in the Atlantic. That number is expected to grow to 30 by the end of 2018, with funding for several more in the pipeline in the years ahead. The average cost to upgrade the ships with the new Lockheed Martin Aegis radar system and the Raytheon SM-3 missiles is roughly $45 million.

The ships serve as both defensive and offensive weapons. The sophisticated SM-3 missiles can zero in on and take out short- to medium-range missiles that might be fired at U.S. or allied forces, and they also carry Tomahawk cruise missiles that can be launched from sea and hit high-value targets or enemy weapons systems from afar, without risking pilots or aircraft.

That dual mission is particularly key now, as the ships in the eastern Mediterranean keep watch on Iran for any possible missile launches but also are ready if the U.S. decides it needs to take action against Syria, where there are ongoing concerns about the government's use of chemical weapons against its own people.

Much like during the 2011 operation in Libya, missiles launched from Navy ships could take out Syrian air defence systems without putting U.S. troops at risk in a ground assault or sending fighter jets into Syrian airspace.

To date, the ships have executed just one missile launch that was not a test. In a mission code-named Operation Burnt Frost, the guided missile cruiser USS Lake Erie sailed out into the Pacific and on Feb. 20, 2008, launched a strike that blew apart a disabled U.S. spy satellite that was tumbling in orbit at more than 17,000 mph (27,357.58 kph) about 130 miles (209.21 kilometres) above the Earth.

The actual target, Navy leaders said, was a spot on the satellite the size of a postage stamp. And the missile, travelling at about 30,000 mph (48,278.08 kph), hit it directly, destroying the satellite's onboard tank of about 1,000 pounds (453.6 kilograms) of toxic hydrazine fuel. Officials had been worried about possible injuries, including from the hazardous fuel, if the satellite came down in a populated area.