The international envoy on Syria Monday urged the country's warring sides not to wait until January 22 peace talks to take steps to rein in fighting on the ground.
"We very strongly appeal to the Syrian government and the opposition to not to wait for the conference," said United Nations and Arab League mediator Lakhdar Brahimi, urging the sides to "diminish the violence (and) release prisoners and detainees of all sorts."
"But being realistic, a lot of the things that need to happen will happen after the conference starts," he told reporters in Geneva.
Brahimi, who spent Monday in closed-door talks with US and Russian officials working on peace plans, spoke after UN Secretary General Ban Ki-moon announced the date of the long-delayed Geneva conference on Syria.
"At long last and for the first time, the Syrian government and opposition will meet at the negotiating table instead of the battlefield," Ban told reporters at the UN's New York base.
"The fighting has raged on far too long -- with more than 100,000 dead, almost nine million driven from their homes, countless missing and detained, and terrible violations of human rights," Ban said, calling the war the "biggest threat to international peace".
"It would be unforgivable not to seize this opportunity to bring an end to the suffering and destruction," he added.
The international community has struggled to broker talks between the regime of Syrian President Bashar al-Assad and the rebels battling him since a bloody March 2011 crackdown on Arab Spring-inspired protests.
The so-called Geneva II conference is meant as a follow-up to one held in June 2012, where world powers called for a Syrian transition government.
But the opposition demanded that Assad stand down before any talks, while the government insisted his future could not up for discussion, and amid spiralling fighting plans for Geneva II repeatedly were put on hold.
Brahimi said the conference would start "without any preconditions".
Russia, Assad's key world-power ally, had pressed the regime to cooperate on talks while the United States, Britain and France worked on the fractured Syrian opposition.
On Monday, US Secretary of State John Kerry said the talks were the "best opportunity" to form a new government.
British Foreign Secretary William Hague, meanwhile, said the run-up to the conference would be a test for the regime, which needed to show it was "prepared to negotiate a political transition and end the violence."
The opposition Syrian National Coalition has agreed to attend, but its authority has been undermined by Islamist rebels.
"The national coalition will play a very important role in forming the delegation," Brahimi said, stressing though that "the delegation has to be credible and ... as representative as possible."
"Not all the people who want to come to Geneva will be able to come, but they should know that this is not an event, this is a process," he underlined, saying that participation in building the "new republic of Syria" would clearly be widened later.
"We are in touch both with the government and the opposition. We are asking them to name their delegations as early as possible, hopefully before the end of the year," said the Algerian veteran mediator who was UN envoy for Afghanistan and Iraq.
"This conference is really for the Syrians to come to Geneva and talk to one and other and hopefully start a credible, workable effective peace process for their country."
Iran is also a key Assad supporter, and Russia has sought to have Tehran involved in talks, raising hackles in the West, which has pushed to involve leading opposition backer Saudi Arabia as a counterweight.
Hopes of headway were raised after world powers Sunday reached a landmark deal with Iran to rein in its controversial nuclear programme.
On the sidelines of the five-day nuclear talks in Geneva, Brahimi met with Iranian Foreign Minister Mohammad Javad Zarif, his Russian counterpart Sergei Lavrov, and top US negotiator Wendy Sherman.
Brahimi said he would meet US and Russian officials again on December 20.
Pressed Monday on whether Iran and Saudi Arabia would be at January's talks, he said: "We haven't established a list yet."
"These two countries will certainly be among the possible participants," he added.