De Mistura has "intensified efforts" towards convening the talks on the target date, hopefully including the "broadest possible spectrum" of opposition representatives, the statement said.
The special envoy on the nearly five-year conflict "counts on full cooperation of all the relevant Syrian parties in this process," it said, adding: "Continuing developments on the ground should not be allowed to derail it."
Deal to evacuate Syrian IS fighters is halted: Hezbollah TV
Last week, the UN Security Council unanimously adopted a resolution endorsing a proposed peace plan to bring the regime and opposition together for talks in January. But no specific date had been given.
The plan is the result of nearly two months of strenuous efforts among top diplomats from 17 countries, including regime backers Russia and Iran. But it does not address the toughest sticking point: the fate of Assad.
The resolution foresees a rapid ceasefire, and calls for talks that would lead to the "establishment of an inclusive transitional governing body with full executive powers" within six months.
Top Syrian rebel leader Zahran Alloush killed in air strike in Damascus suburb
Syria's government said Thursday it was ready to take part in fresh talks aimed at ending the war, which has killed more than 250,000 people, but appeared to make its participation conditional on which opposition groups attend.
De Mistura said he was relying on "the continued crucial support of the International Syria Support Group," which also includes the United States.
The UN resolution enshrines the plan developed by the ISSG countries in a series of meetings in Geneva and Vienna.
Saudi-led coalition: Pakistan won’t support moves against Syria or Iran, say officials
"The people of Syria have suffered enough. Their tragedy is now felt throughout the region and beyond," the statement from de Mistura's office said.
"They deserve the full attention and commitment from all their Syrian representatives, who should now show leadership and vision to overcome differences for the sake of Syria."
COMMENTS
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ