Several Arab countries have offered to join the United States in air strikes against Islamic State targets, U.S. officials said on Sunday, indicating a possible widening of the air campaign against militants who have seized parts of Iraq and Syria.

The officials declined to identify which countries had made the offers but said they were under consideration as the United States begins to identify roles for each country in its emerging coalition against jihadists who have declared a caliphate in the heart of the Middle East.

The addition of Arab fighter jets could strengthen the credibility of the American-led campaign in a region skeptical of how far Washington will commit to a conflict in which nearly every country has a stake, set against the backdrop of Islam's 1,300-year-old rift between Sunnis and Shi'ites.

"I don't want to leave you with the impression that these Arab members haven't offered to do air strikes because several of them have," a senior U.S. State Department official told reporters in Paris.

The official said the offers were not limited to air strikes on Iraq. "Some have indicated for quite a while a willingness to do them elsewhere," the official said. "We have to sort through all of that because you can't just go and bomb something."