The troops are expected to move into the Gulf in coming weeks as part of a rolling commitment to destroying Saddam Hussein's regime, according to US officials.
They said the troop movements had always been part of the plan and denied they were sparked by unexpected Iraqi resistance.
There are currently around 90,000 US ground troops inside Iraq and around 250,000 US military personnel in the Gulf, according to US officials.
Britain has a further 45,000 personnel in the Gulf.
The new troops, if all are deployed, would see the number of US ground forces in Iraq doubled and would bring the total US presence in the
region to around 350,000, including those on ships, air bases and command centres.
Of the troops arriving in the next few weeks, some from the US Army's 30,000-strong 4th Infantry Division have already begun flying out of Fort Hood, Texas, to Kuwait. The 1st
Armoured Division in Germany, the 1st Cavalry Division at Fort Hood, the 2nd Armoured Cavalry Regiment at Fort Polk, Louisiana, and the 3rd Armoured Cavalry Regiment at Fort Collins, Colorado, are expected to deploy soon.
But equipment for the 4th Division is not expected to arrive in Kuwait by sea for around two weeks.
US officials denied that this deployment signalled a lengthier war or that they had become bogged down.
They also denied that the need for protection of their extended supply lines had been underestimated.
Also, many Allied military personnel are expected to be assigned to remain in Iraq for some time after Saddam's regime is unseated.
Lieutenant General William Wallace, Commander of the US Army V Corps and the man who will lead the attack on Baghdad, told the Washington Post: "The enemy we are fighting is different from the one we'd war gamed.
"We knew they were there the paramilitaries but we didn't know they would fight like this."
But a senior official at CentCom said they had always expected Iraqis to employ ruses such as dressing up in US and British uniforms.
In the 1991 Gulf War a commitment of half a million personnel was needed to get Saddam Hussein out of Kuwait.