"Between now and September we have a window to find a formula which combines the fights against Daesh and Al-Nusra and the political transition," Staffan de Mistura said, using an alternative term for the Islamic State and also referring to the Al-Qaeda affiliate Al-Nusra Front.
The comment, made following a Rome meeting between de Mistura and Italy's Foreign Minister Paolo Gentiloni, appeared to push back an August 1 deadline set for political transition by the international group backing the Syrian peace process.
"It is a crucial moment," he said, adding that "the key lies in a possible agreement between Russia and the United States".
"They have proved themselves to be able to agree and the others will adapt," he said, referring to previous successes such as a truce in February between regime and non-jihadist rebel groups which was brokered by Russia and the United States.
Gentiloni said Russia had made a commitment but "we expect more".
Talks on the political transition have stumbled over the fate of Syrian eader Bashar al-Assad with the opposition insisting that he must quit.
The West has accused Moscow of targeting the opposition in an effort to prop up Assad's regime.
"The future of Syria cannot be managed by Bashar al-Assad," Gentiloni said.
"When negotiations resume there must not be preconditions but a certainty that the future is not in his hands".
The UN-backed talks, aimed at ending a war which has left more than 280,000 people dead and driven millions from their homes, are in theory set to resume this month.
However de Mistura has previously said he wants guarantees of progress efore re-starting them.
The last round of talks held in Geneva ended in late April without any breakthrough.
Damascus and Moscow are continuing air strikes against "terrorist"
positions while the US-led coalition against IS is bombing jihadists and backs armed groups on the ground.