The UN envoy for Syria said Wednesday he was still aiming to resume peace talks in July but was first seeking guarantees that there will be progress on plans for a political transition.
"We are aiming within July but not at any cost and not without guarantees," Staffan de Mistura said following a meeting of the Security Council.
The UN-backed talks are aimed at reaching a political settlement to Syria's five-year war, which has left more than 280,000 people dead and driven millions from their homes.
The last round of talks held in Geneva ended in late April without any breakthrough as fighting flared on the ground despite a ceasefire that had a promising start when it was reached in February.
De Mistura said he was working to ensure that "when we call the talks there is possibility of moving forward on political transition by August," a deadline set by the international group backing the Syrian peace process.
Most diplomats agree that the August 1 target date to agree on a political transition will be missed, but De Mistura refused to downplay chances for progress.
"Let's talk again in August," he told reporters who suggested prospects were dim.
"There is a lot of discreet diplomacy that is taking place while we are talking," he added.
Returning from Saint Petersburg where he held talks with Russian leaders, De Mistura will on Thursday travel to Washington for further discussions on laying the groundwork for peace talks to resume.
De Mistura said the Syrian government and the opposition must return to the table "with a feeling of urgency and work on some ideas on how to bridge their differences between what everyone means by political transition."
Talks on the transition have stumbled over what role Syrian leader Bashar al-Assad will play, with the opposition insisting that he must leave power immediately.
A UN roadmap calls for a transitional body to be set up in Damascus by August 1, followed by the drafting of a new constitution leading to elections 18 months later.
The UN envoy suggested that time was running out for progress to be achieved, with US President Barack Obama set to step down at the end of the year and UN Secretary-General Ban Ki-moon wrapping up his term.
"I am putting pressure on all parties because I think that the main parties who can make this happen need to feel that they have a historic responsibility," he said.
He pointed to the UN General Assembly meeting and a G-20 summit in China, both in September, as gatherings that will allow world powers to take stock of the peace effort in Syria and decide on the way forward.