"We could breathe freely again after the Idlib deal brokered by Turkey and Russia, which prevented a massacre," European Union's foreign policy chief Federica Mogherini said late Wednesday.
"I would like to thank Turkey and Russia for their efforts," she added.
Saying that the war in Syria is not over yet, she added that "there won't be winner in the aftermath."
Turkey and Russia have signed a memorandum of understanding that calls for preserving the de-escalation zone in Idlib.
According to the agreement reached in Russia's Sochi, opposition groups in Idlib will remain in areas where they are already present, while radical groups will lay down their arms and leave the region. Under the pact, Russia and Turkey will carry out joint patrols in the area.
President Recep Tayyip Erdoğan and U.N. Secretary-General Antonio Guterres discussed measures taken for the de-escalation zone in Idlib as well as the political process in Syria earlier this week on the sidelines of the General Assembly session.
The province of Idlib is the largest and last opposition stronghold. As of recently, Turkey has been eager to prevent a potential regime offensive there to avoid a humanitarian disaster.
Mogherini also said Wednesday a so-called Special Purpose Vehicle (SPV) under consideration to facilitate trade with Iran could be in place "before November."
The SPV aims to keep trade flowing even if possible U.S. sanctions hit Tehran. Asked at an event in New York when it could be established, Mogherini said: "I believe before November."
European diplomats have described the SPV proposal as a means to create a barter system, similar to one used by the Soviet Union during the Cold War, to exchange Iranian oil for European goods without money changing hands.