The coalition spokesman in Iraq, Dan Senor, said the Spanish presence was critical to their efforts.
By Paul Wood
BBC correspondent in Baghdad
Speaking shortly after Spain's Socialist Party prime minister-elect confirmed he will pull troops out of Iraq unless the UN takes charge, Mr Senor said the Spaniards were performing "heroically".
Spain has the fifth largest national contingent in Iraq
However, his remarks do not distinguish between the military and the political effects of a Spanish withdrawal.
Spanish troops make up less than 1% of the total international military strength in the country.
The Spanish Ministry of Defence says it has 1,412 military personnel currently serving in Iraq. Most are based in Diwaniya in southern Iraq.
Ahead of the Spanish, the coalition includes, in order of size, 124,000 Americans, 8,000 British troops, some 2,500 Poles, and 1,600 Ukrainians.
So, valuable and professional as the Spanish contribution undoubtedly is, their withdrawal is not going to cause a collapse in security.
The political effects, however, are more worrying for the US and Britain - the two occupying powers here.
The coalition now comprises some 60 nations. Washington repeatedly stresses this figure to emphasise that this is not a purely American operation.
If the Spanish do leave, will others reassess their commitment?
And the Spanish have said they will make their decision based on the situation in Iraq up until 1 July, when a new interim Iraqi government will take over.
The Americans know they have to maintain confidence if this transfer of power is to go ahead - the last thing they want in this very sensitive period is for this political process to be undermined by Spanish doubts over security.