"From a dramatic and terrible conflict, we run the risk of even a global conflagration," she said in an interview with news channel SkyTG24.
"This is how it always begins. Limited strikes without a UN mandate. Syria will obviously react," said Ms Bonino, a former European aid commissioner.
"Even though it seems slower, tougher and sometimes looks like it won't succeed, keeping up diplomatic and political pressure is the only solution," she said.
Ms Bonino has ruled out any Italian participation in strikes against Syria over the alleged use of chemical weapons by Syrian President Bashar al-Assad's regime.
Italy played a pivotal role as a launch pad for air strikes on Libya but government sources quoted in Italian media said it would likely not allow third countries to use its bases if asked to do so now.
Ms Bonino is a veteran campaigner in Italy known for her struggle for civil rights and pacifist causes.
Earlier this week she warned: "Even the option of a limited intervention risks becoming unlimited".