"We are deeply concerned about the worsening security situation in Ivory Coast and urge enhanced efforts to stop the country's slide into full-scale civil war, which would likely involve ethnic cleansing and other mass atrocity crimes," the ICG said in an open letter to African leaders Tuesday.
The U.N. Security Council last week expressed "deep concern" over reports that at least 30 civilians were killed and 60 more were wounded when forces loyal to Gbagbo shelled a market in Abidjan.
ICG said the conflict is escalating rapidly and called on ECOWAS to prepare to remove Gbagbo from power "by all necessary means before it is too late."
The Security Council recognized Gbagbo's rival Alassane Ouattara as the winner of a November presidential election but the ICG said the measure did little to prevent an outbreak of violence.
All of West Africa faces a risk of destabilization if the situation in Ivory Coast descends further, the ICG said.
"Ivory Coast is no longer on the brink of civil war," writes ICG President Louise Arbour. "It has already begun."