US President George W Bush has expressed regret to Spain's new Prime Minister, Jose Luis Rodriguez Zapatero, over his decision to withdraw 1,400 troops from Iraq.
The two leaders have held a telephone conversation.
A White House spokesman said that Mr Bush warned Spain against taking further actions that could give 'false comfort to terrorists'.
The deputy coalition director of military operations in Iraq, Brigadier General Mark Kimmitt, has said the planned withdrawal of Spanish troops will not create a 'security vacuum'.
Mr Zapatero said the Spanish soldiers would be recalled because there was no sign that his terms for their continued participation had been met.
It follows the admission by the US administrator in Iraq, Paul Bremer, that security forces would not be able to ensure security in Iraq by the June deadline for the transfer of sovereignty to the Iraqis.
The US has said it expects other coalition nations to reassess their participation in Iraq.
Bid to end Fallujah stand-off
Meanwhile, the US-led coalition in Iraq has announced measures to end the military stand-off in the city of Fallujah, west of Baghdad.
These include shortening the curfew and allowing unfettered humanitarian access to the people of Fallujah.
Coalition spokesman Dan Senor said it wanted the surrender of all heavy weapons held by Sunni militants.
The development comes after days of talks to resolve the stand-off.
US forces staying outside Najaf
In a separate development, US forces positioned outside the Iraqi city of Najaf, where the radical Shia cleric Moqtada al-Sadr is believed to be hiding, have said they will not attempt to enter the city for the moment.
Hundreds of Iraqis have been killed by the American offensive against the city, which is one of the most sacred cities in Shi'ite Islam.
Since the start of this month, more than 100 US soldiers have also been killed. The US army has said it wants to kill or capture al-Sadr.
The cleric has called on his followers to stop attacks on Spanish troops following the announcement of the troops' withdrawal as soon as possible.
