Zapatero, who withdrew Spanish troops sent to Iraq by his conservative predecessor Jose Maria Aznar shortly after taking office in April, said on Thursday it was vital UN members established a consensus on Iraq by burying old differences.
The Spanish leader reiterated that it was up to the UN to organise any international conference on the future of Iraq, which - under a French proposal - would include representatives of the armed opposition.
"An initiative outside the aegis of the United Nations will not have positive repercussions. Within the United Nations we will be able to decide who participate at the conference," Zapatero said, adding the situation in Iraq "is getting worse daily".
"Things are going badly in Iraq because we didn't go through the United Nations. Things are better in Afghanistan because we did go through the UN," Zapatero said.
"Spain does not want an initiative which opens old wounds but wants to see a consensus where by a path is opened towards stability and democracy in Iraq," he said.