Mr. de Mistura on Friday offered the first hints of how the pause in fighting is to work — at least in theory.

The Syrian government, plus Russia, have agreed to end aerial bombardments “against the armed opposition groups and parties to the cessation of hostilities,” he said.

Those who have signed on to the deal — both government and rebel — have also agreed not to fire at each other or to advance into new territory, and to allow aid agencies to deliver food and medical supplies into all Syrian cities and towns, Mr. de Mistura added. The United Nations maintains that government forces and the Islamic State are responsible for besieging most of the 487,000 people besieged for months and unable to get basic relief.

His comments revealed the pitfalls too in how the truce will be monitored. A “task force” led by Russia and the United States is to delineate which parts of the country are held by the banned terrorist groups that are not part of the cessation of hostilities. It is to gather information on truce violations and “defuse tensions.”

Already there seemed to be tensions about how the truce would work.

The main Syrian opposition bloc, which calls itself the High Negotiation Committee, has agreed only to a two-week pause in fighting and wants Russian bombardments to stop.

The Russian deputy foreign minister, Gennady Gatilov, said flatly on Friday that the combat against the Islamic State, Nusra Front and what he called “other terrorist groups” would continue, and went on to criticize Turkey for what he called “illegal flows” of fighters and weapons into Syria. It was a measure of how tensions between Russia and Turkey have increased up in recent months, posing a new challenge for any effort to end the war in Syria.

Turkey is in its own fight with Syrian Kurdish groups in the border region, including the Y.P.G., which is supported by the United States. A spokesman for President Recep Tayyip Erdogan of Turkey said on Friday that the government had doubts about the truce’s viability.