See more of the story

GENEVA — The U.N. envoy for Syria hosted key diplomats from Iran, Russia and Turkey on Tuesday to discuss work toward rewriting the country's constitution, amid concerns about a possibly devastating military offensive on rebel-held Idlib province.

The talks led by Staffan de Mistura started and ended with little or no comment to reporters at the U.N. offices in Geneva, and offered a sideshow to the concerns about a looming battle for the northern province — the last remaining rebel stronghold in Syria after 7½ years of war and now home to some 3 million civilians.

De Mistura's spokesman, Michael Contet, said in an email that any debriefing by the envoy about the meeting will be "reserved" for comments that he plans to make to U.N. Security Council next Tuesday.

Turkey's Foreign Ministry said the diplomats discussed the formation of the constitutional committee, "which constitutes a significant step in the struggle to find a political solution to the Syrian crisis," as well as procedural rules.

It said the sides confirmed their "agreement in principle" to lists of participants proposed by the Syrian government and the opposition and held consultations on which civil society groups would also participate in the committee.

The ministry said the Turkish, Russian and Iranian officials would hold more talks on the issue at a "technical level."

On Monday, the head of the U.N. humanitarian agency, Mark Lowcock, warned that Idlib could see "the worst humanitarian catastrophe, with the biggest loss of life of the 21st century."

Iran and Russia have backed a military campaign on Idlib involving Syrian President Bashar Assad's forces, despite Turkey's pleas for a cease-fire.

Before Tuesday's meeting, Hossein Jaberi Ansari, a special envoy for Iran's foreign minister, said a "good result" could emerge. Asked whether Iran shared the concerns about a possible humanitarian catastrophe in Idlib, Jaberi Ansari replied: "We are worried too. We are trying to avoid this."

Russian President Vladimir Putin's special envoy for Syria, Alexander Lavrentiev, declined to answer a question on his way into the talks about whether Russia would stop its airstrikes.

De Mistura met informally with members of the three delegations on Monday.

The talks are set to focus on creating a constitutional committee under Syria's Russian- and Iranian-backed government. Russia, Turkey and Iran have been working together as "guarantors" for a series of talks around ending Syria's war. Turkey has taken in 3.5 million refugees from its neighbor.

On Monday, airstrikes on Idlib and Hama provinces forced some people to flee their homes, according to the Britain-based Syrian Observatory for Human Rights.