 Thousands of civilians have been displaced by the fighting in Falluja |
The move is at the request of military commanders on the ground.
BBC Pentagon correspondent Nick Childs says it marks a sharp reversal of strategy, and highlights their concern about the escalating violence in Iraq.
The deployment comes as US forces continue to attack insurgent targets in the restive city of Falluja.
The predominantly Sunni Muslim city, 50km (30 miles) west of Baghdad, has spearheaded opposition to the US-led occupation of Iraq.
United Nations Secretary General Kofi Annan has warned that American military action in Falluja could be fuelling the resistance to the US-led occupation of Iraq.
But US President George W Bush said the military would take whatever action was necessary to secure the city.
Casualty rate
The reinforcements being sent to Iraq include 28 of the giant M1 Abrams tanks.
The deployment reflects the inadequacy of the standard military personnel transporter in Iraq, the Humvee, to deal with roadside bombs and rocket-propelled grenades.
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April has seen more than 120 US troops killed in Iraq - by far the highest casualty rate since the toppling of Saddam Hussein a year ago.
Our correspondent says that many of the US units now in Iraq have deployed only in recent months.
They were sent without much of their heavy equipment because at the time it was thought that lighter, more mobile forces were what was needed as the security situation appeared to be improving.
With the upsurge in fighting, he adds, US marine and army commanders urgently requested more firepower and protection for their troops.
"That armour is either in Iraq now or is arriving as we speak. So those requests were quickly filled," US Marine Corps Major General John Sattler told reporters.
Air bombardment
Recent days have seen US air strikes against insurgents in Falluja, which has been encircled by a force of US marines.
Jet aircraft, helicopters and AC-130 gunships have bombed and strafed targets in several districts of the city.
Marine commanders maintain that they have called in air support only after coming under enemy fire.
They say a ceasefire agreed earlier this month is still valid, but that they will respond with overwhelming force when fired upon.
 Satellite image showing location of recent fighting in Falluja  |
Enlarge ImageCaptain Christopher Logan, a spokesman at the US Marines base just outside Falluja, said: "We ceased fire and they kept firing. If they are stupid enough to attack us, we'll destroy them."
The US military says it is holding back from an all-out assault on the city of 300,000 people, in the hope that the insurgents might yet agree to turn in their heavy weapons.
Talks between US forces and local leaders to secure a solution continue alongside the fighting.
Gen Sattler said there were "somewhere around 1,500" guerrillas in Falluja, including former members of Saddam Hussein's Republican Guard military units and foreign Islamic militants.
Speaking in Washington, President Bush said: "Our military commanders will take whatever actions necessary to secure the city."
He added that there were "pockets of resistance" in the city but "most of Falluja is returning to normal".
Secretary General Annan, whose organisation is helping form an interim Iraqi government in line with US plans to transfer power on 30 June, urged US forces to show restraint.
"The more the occupation is seen as taking steps that harm the civilians and the population, the greater the ranks of the resistance grows," he told a news conference in New York.
Doctors in Falluja say some 600 people have been killed since the siege began three weeks ago.
Thousands have fled the city, but many are now attempting to return, despite the fighting.
An unconfirmed report says US marines manning a checkpoint opened fire on a minibus on Thursday morning, killing four civilians.
The US military operation in Falluja began on 5 April following the gruesome killings of four American civilian contractors there.
'Support dwindling'
Elsewhere in Iraq, two US soldiers have been killed in separate attacks by guerrillas.
The US military said one died in a rocket-propelled grenade attack in eastern Baghdad, while the other was killed when a roadside bomb exploded near a convoy close to the town of Baquba, 40km (24 miles) north of the capital.
Thursday also saw a foreign civilian, possibly a South African, killed in a drive-by shooting in the southern city of Basra.
In the US, a new opinion poll suggests dwindling support among Americans for the invasion and occupation of Iraq.
Just 47% of 1,042 Americans questioned believed invading Iraq was the right thing to do, the lowest support recorded in the polls since the war began.
