Defence officials in Washington are quoted as saying there are plans to move another 50,000 troops next month to join the 60,000 based in the Gulf and Turkey.
UK Prime Minister Tony Blair told the country's armed forces on Friday that they, too, must be ready for a possible confrontation with Iraq.
In his first public response to Iraq's weapons declaration, George W Bush told reporters on Friday that it was "not encouraging".
"We expect Mr Saddam Hussein to disarm," Mr Bush said.
"Yesterday was a disappointing day for those who long for peace," he added.
Baghdad says the initial US response to the declaration "exaggerated".
"Even before they were able to read and analyse the declaration they said it had many gaps," General Hussam Mohammad Amin, the chief Iraqi officer liaising with the UN weapons inspectors, told the Reuters news agency on Friday.
The United States said on Thursday that Baghdad was in "material breach" of a critical United Nations resolution - which Washington says gives it the authority to attack Iraq.
Other UN Security Council members, including the UK, Washington's closest ally, have not supported that US interpretation of the resolution.
'Prepare for war'
In his Christmas message to the British armed forces, Mr Blair told troops to prepare for war.
"They key thing at the moment is to make sure... that we are able to undertake this mission if it falls to us to do so," he said in a recorded message broadcast on Friday.
The increase in military pressure on Iraq followed the first report by the UN's chief weapons inspector on Iraq's weapons declaration.
Hans Blix told the Security Council on Thursday that the document contained little new information.
He has called on the United States and Britain to hand over intelligence relating to sites where they believe Iraq has weapons of mass destruction.
Mr Blix told the BBC that, if he knew where they thought Iraq was storing banned materials, he could send his inspection teams to check.
He said Western governments had intelligence sources not available to the United Nations - such as spies - but he was not receiving as much support as he would like.
Scientists
The US is also putting forward its own proposals to Mr Blix, calling for inspectors to bring Iraqi scientists out of the country so they can be interviewed about Baghdad's weapons programmes.
Both Britain and the US have said Iraq's 12,000-page arms declaration is incomplete.
IRAQI MATERIAL UNACCOUNTED FOR
Mr Blix told the UN Security Council that Iraq's declaration did not contain the necessary evidence that known weapons of mass destruction had been destroyed.
He will make his report on the weapons document on 27 January.
Baghdad produced its document in accordance with a Security Council resolution passed last month which threatens "serious consequences" if Iraq fails to comply with disarmament demands.
Mr Blix said he had asked for help from US Secretary of State Colin Powell and UK Foreign Secretary Jack Straw and hoped a list of suspect sites would come.
Mr Powell has said the US will provide inspectors with "every possible" assistance, and a BBC correspondent in Washington says this is likely to include high-quality intelligence.
Clear position
Mr Powell said omissions in the document meant Baghdad was "well on its way to losing this last chance" to avoid military action.
He said there were clear issues that needed to be addressed in the coming weeks.
Mr Blair used his Christmas message to the military to say no-one knew if war in Iraq would be necessary, but preparations had to be made, including the build-up of troops.
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