Peace talks between the Syrian government and opposition groups are to start on Friday, the UN special envoy on Syria said.
The talks in Geneva are expected to take six months and the sides will not talk directly to each other to begin with.
Staffan de Mistura said the priorities would be creating a broad ceasefire, stopping the threat from the Islamic State (IS) group and clearing the way for humanitarian aid.
"We want to make sure that when and if we start, to start at least on the right foot," he said. "It will be uphill anyway."
Turkey's foreign minister warned that any participation of Kurdish forces in the Geneva talks would be dangerous and would spell the end of the initiative seeking to end the nearly five-year conflict.
Mevlut Cavusoglu said Turkey considers the Syrian Kurdish forces "terrorists", accusing them of co-operating with Kurdish rebels who are banned in Turkey. He said they have no place among the opposition at the Geneva talks.
Geopolitical tensions between countries including Turkey, Russia, Iran and Saudi Arabia have weighed heavily on efforts by negotiators.
The initiative has run into delays and disputes notably over the invitation list. Fierce ongoing tensions have also led negotiators to decide that the opposing sides will not initially meet face-to-face - a sign that even minimal progress is far from certain.
The Geneva talks are the first since discussions collapsed two years ago.
Russia has called for the inclusion of Kurdish representatives, and the US and others have supported the Kurds in the fight against IS in Syria. But Turkey is strongly opposed.
"There are efforts among some countries to water down the opposition. We oppose this," said Mr Cavusoglu. "To insist that terror groups such as the YPG (the main Kurdish militia) are included within the opposition would lead to the failure of the process. We have to insist that this is extremely dangerous."
But European Union foreign policy chief Federica Mogherini, who spoke to reporters alongside Mr Cavusoglu, said that "from the European perspective, we attach an extremely high value on the fact that the process in Geneva will be inclusive".
The UN Security Council passed an ambition resolution on the Syria crisis that set a target for the peace talks to start this month. That resolution also aims to produce credible governance and a schedule for drafting a new Syrian constitution.
But air strikes by Russia - a key backer of Syrian president Bashar Assad - against rebels have altered the military situation on the ground.