BEIRUT, Lebanon — A resurgence of deadly attacks by pro-government forces in so-called de-escalation zones in Syria, including a triple airstrike on a busy marketplace that killed more than 50 people, is undermining an agreement portrayed by its sponsors as a crucial step toward ending the six-and-a-half-year civil war.

The accord — reached this year between Russia and Iran, which are allied with Syria’s government, and Turkey, which backs some rebel groups — established four de-escalation zones where attacks were supposed to decrease and so help pave the way for a peace settlement.

The de-escalation zones encompass most of the remaining areas of Syria still held by insurgent groups, not including the Islamic State. Under the agreement, the combatants are to refrain from new attacks, except against hard-line groups that have not signed on to the pact. Longstanding sieges are to be lifted to allow free movement of goods and people.

But siege and bombardment tactics, mostly by the Syrian government against rebel-held areas, have continued despite the Astana accord, named for the Kazakh capital, where it was struck. The recent uptick in attacks — and the lack of any outcry from international sponsors of the Astana deal — has bolstered skepticism from opponents of the Syrian government who doubted the deal’s good faith from the start.