There are currently two types of cruise missile in service - the air-launched AGM-86 and the Tomahawk BGM-109 ship or submarine-launched version.

Whether armed conventionally or with nuclear warheads, both are highly sophisticated all-weather land-attack weapons.

They are known as a cruise missiles because of the small turbofan engines - similar to those found on commercial airliners - which they use to cruise to their targets.

During launch, a solid propellant rocket fires the Tomahawk version to sufficient altitude. The turbofan engine then takes over for the cruise portion of flight.

Air launched cruise missiles have a range of 1,500 miles

Cruise missiles are very effective because they are difficult to detect. They have a small cross-section, and fly at very low altitudes.

Infrared detection is difficult because turbofan engines emit little heat.

The air-launched version is 20 ft, 9 ins (6.29 m) long, weighs 3,150 lbs (1,417.5 kgs) and has a range of about 1,500 miles (2413 kms).

The sea-launched missiles are slightly smaller 18 ft 3 ins long (5.56 m) and 20 ft 6 ins (6.25 m) with solid-fuel booster.

Each weighs 2,650 lbs (1192.5 kgs) and has a range of 690 miles (1110 kms).

Versatile

The total cost of the Tomahawk missile programme is estimated at $11.21bn. They can be fitted with single or multiple warheads.

Sea-launched missiles enable the US to strike deep into enemy territory

Each missile comes with a terrain contour matching radar, which compares a stored map reference with the actual terrain to fix the missile's position.

The computer can correct the missile's course, enabling it to fly at very low, landscape-hugging altitudes, and follow a complicated route.

As the missile nears its target, the Digital Scene Matching Area Correlation system takes over. This compares a stored image of the target with the actual target, enabling a very high level of accuracy.

The US is currently working on a version of the missile which can be re-targeted mid-flight.

It would also be able to circle its target for several hours and with on-board TV cameras, allow commanders to assess damage to the target, and, if necessary redirect the missile to another target.

During the 1980s, Ground Launched Cruise Missiles (GLCM) fitted with nuclear warheads were stationed at the US airbase at Greenham Common in the UK, causing a storm of protest.

GLCM's were eventually scrapped as part of the terms of the Intermediate Nuclear Forces (INF) Treaty signed between the United States and the former Soviet Union.

Mainstay

The missiles have however remained in use with the US Air Force, carried by giant B-52 bomber aircraft.

They also form the backbone of the US Navy's firepower against targets deep within enemy territory, including air defence and communications.

Stationing cruise missiles in the UK brought fierce protests

Cruise missiles were used extensively during the Gulf War in 1991, and in follow-up operations against Iraq in January and June 1993.

They were also used in Bosnia (Operation Deliberate Force) in 1995 and in Iraq again (Operation Desert Strike) in 1996.

Former president Bill Clinton launched cruise missile strikes against training camps run by Osama Bin Laden's al-Qaeda network in Afghanistan in 1998.

Cruise missiles were also fired during the air campaign over Kosovo in 1999.