During the visit to Italy, the State Department had announced, Kerry would have the chance to meet with leaders of the Syrian opposition that have battled the regime of Syrian President Bashar al-Assad for two years, in a civil war that has claimed roughly 70,000 lives.

Over the weekend, however, the Syrian Opposition Council — the main umbrella group for the broad array of rebel politicians, clerics and soldiers, many of them Islamists with ties to al-Qaeda and other unsavory networks — announced it was boycotting the session with Kerry in Rome.

The Syrians complained that the United States — which has blacklisted al-Nusra, the al-Qaeda-linked group that has emerged as one of the most effective combat brigades in the battle against Assad’s military — has not been even-handed enough in its treatment of the rebels.