"Iraq should not misunderstand our determination," Mr Cohen said.

Mr Cohen said a second carrier battle group led by the USS Carl Vinson is on its way to the region. Additional ground troops are going to Kuwait.

An air force expeditionary wing made up of over 30 assorted aircraft is also being despatched to the Gulf.

Mr Cohen said there have been no American casualties so far. US officials cannot calculate Iraqi casualties, he said.

Strategic sites in Baghdad

Secret targets

The US Joint Chiefs of Staff, General Hugh Shelton said he was unable to reveal the targets of the military operation at this stage.

Earlier a Pentagon official said the strikes were expected to target Iraq's elite Special Republican Guards, air defences, command and control system and facilities associated with weapons of mass destruction programs and the means to deliver them.

The aircraft carrier USS Carl Vinson was in the northern Indian Ocean on Tuesday and is scheduled to arrive in the Gulf by December 18, joining the carrier USS Enterprise.

Its arrival will roughly double the number of carrier-based combat aircraft available for an attack to about 100.

The carriers carry F/A-18 Hornets, F-14 Tomcats and EA-6B electronic warfare jets.

Formidable fire power

The BBC Defence Correspondent Jonathan Marcus says that the United States already has a formidable array of fire power in the Gulf.

Because the Americans had planned a pre-Christmas changeover of their forces, those available at the moment are effectively doubled.

He added there is no doubt the United States can administer a very damaging blow to Iraq's military infrastructure, although inevitably, this would lead to significant Iraqi casualties.

A Pentagon spokesman said on Monday there were 24,100 US troops in the Gulf, 201 military aircraft and 22 naval ships, including eight warships capable of firing Tomahawk cruise missiles.

Before the bombing began, US forces in the region include:

• The USS Enterprise aircraft carrier, patrolling Gulf waters with an accompanying naval battle group armed with several hundred cruise missiles

• A total of 21 ships including eight capable of firing Tomahawk cruise missiles

• More than 200 aircraft, including B52 bombers and B1 stealth bombers, some based on the Indian Ocean island of Diego Garcia

• Some 24,000 service personnel, including 2,400 army personnel who make up part of the US task force in Kuwait

Britain is giving the US the most military support. The UK has 12 tornado aircraft in Kuwait, six Jaguar bombers in Turkey, which could be used in bombing raids.

Britain has also deployed ships and military personnel in the Persian Gulf.

Iraq's military strength

It is thought that by the end of the Gulf conflict only a quarter of the Iraqi army's pre-war strength remained.

But it has since been rebuilding its forces and troop strength is now thought to be about 400,000.

• The Iraqi air force has around 580 combat aircraft, including 50 MiG-23 multi-role fighters, MiG-21s, MiG-25s, MiG-29s, Sukhoi Su-17s, Su-20s and Su-25s, and some Mirage planes.

• Iraq's Navy was largely destroyed in the Gulf War and is unlikely to pose much of a threat. It is thought to comprise two frigates and a small number of patrol aircraft.

• Iraq has been able to rebuild its air defence system that was crippled in the Gulf War. But while it still has highly capable mobile missile systems like the SAM-6, the flexibility of its overall air defence network is probably limited.

• Iraq is still believed to have some modified Scud missiles equipped with chemical or biological warheads, with a range of up to 450 miles (700 km).

Since the end of the Gulf War in 1991, the United Nations has tried to uncover the true extent of Iraq's weapons of mass destruction, and its stock of long-range missiles.

Iraq is known to have produced chemical weapons like mustard gas and a deadly nerve gas called VX.