“Iran is currently a major actor with respect to adverse consequences in Syria,” Mr. Kerry said then.

But with the Islamic State terrorist group making gains in Syria, a tidal wave of migrants swamping Europe, no formal peace talks in sight and Russia engaged in a military buildup at an air base near Latakia, on Syria’s Mediterranean coast, Mr. Kerry is now reaching out to Iran, which has been a major backer of the Syrian president, Bashar al-Assad, to see if there is a basis for resuming negotiations. Mr. Kerry has also been consulting with Russia, European nations and Arab states.

Mr. Kerry’s meeting with Mr. Zarif was his first since six world powers and Iran reached a nuclear accord in July. While American officials discussed Syria with the Iranians on the margins of the nuclear talks, they saw the Saturday meeting as a chance for a fuller discussion.

Still, the conditions for a breakthrough on Syria are not auspicious. The Obama administration’s ability to shape a diplomatic outcome in Syria has been diminished by the Pentagon’s failure to train and equip more than a handful of moderate Syrian rebels to confront the Islamic State.

At the same time, Russia has expanded its influence with its military buildup at a base near Latakia. Mr. Kerry said this month in London that Mr. Assad had shown no interest in negotiating a political transition in which he would eventually step down and that Russia had done nothing to bring him to the table. Iran has long had a strategic interest in maintaining Mr. Assad in power because the airport in Damascus, the Syrian capital, serves as a channel for shipping Iranian weapons to Hezbollah, the Lebanese Shiite militia. Iran pressed Hezbollah to join the fighting in Syria on the side of Mr. Assad, provided weapons to the Syrian government and sent its own Quds Force personnel there.