In a statement at the United Nations in New York on Tuesday, the Organisation of Islamic Conference also stressed the right of the Iraqi people to fully control their natural resources and "establish a fully representative and broad-based government".
The statement will be a psychological boost to Baghdad's US and British rulers, who have been under fire from the Muslim world since invading Iraq in March.
But it will go down like a lead balloon in parts of the Muslim world where many are against giving the American-led occupation legitimacy.
Foreign ministers from the 57-nation OIC called on group members to provide "all forms of support and assistance to meet Iraq's needs" so the war-torn country can quickly get back on its feet.
But it also "condemned with deep sorrow" alleged mass killings of innocent Iranians and Kuwaitis as well as Iraqis, carried out by Iraq's former rulers.
"The meeting called for bringing to justice the officials of the former Iraqi regime who committed such crimes," the OIC statement said.
Iraq's 25-member governing council was appointed in July by US administrator Paul Bremer, who can veto its decisions.
Council members have been pressing Washington to give it greater leeway to administer their oil-rich nation.
US Secretary of State, Colin Powell, said last week he wanted to see an Iraqi constitution ready for ratification in about six months.
But the Iraqi group working on the new constitution said it would be impossible to complete the job within that time because of deep disagreements over major issues.