The truce under discussion was meant to begin Friday, but has failed to materialise so far, with the UN's Syria envoy also acknowledging that a proposed February 25 date to restart stalled peace talks was no longer "realistically" possible.
Meanwhile, tensions between regime backer Moscow and opposition supporter Ankara escalated, with Russia convening a UN Security Council meeting for later Friday to discuss the possibility of a Turkish ground intervention in Syria.
French President Francois Hollande said Ankara's escalating involvement in the conflict was creating a risk of war between Turkey and Russia.
"Turkey is involved in Syria... There, there is a risk of war," Hollande told France Inter radio. "That is why the (UN) Security Council is meeting."
And Saudi's Foreign Minister Adel Jubeir raised the prospect that Syrian rebels could be supplied with surface-to-air missiles, though he said it was not a decision Riyadh would take alone.
The US-Russian meeting in Geneva brought together military officials and diplomats, Russia's foreign ministry said.
It is intended to pave the way for a broader meeting with participation by UN peace envoy Staffan de Mistura.
On the ground, the Kurdish-led Syrian Democratic Forces alliance seized the town of Al-Shadadi from IS, with backing from US-led air strikes, a monitor and Kurdish sources said.
The Britain-based Syrian Observatory for Human Rights said SDF fighters were carrying out "mopping up" operations around the town in Hasakeh in northeast Syria.
Al-Shadadi was the largest town controlled by IS in the province, and the defeat extends earlier losses for the jihadists there.
- Turkey, Russia tensions spike -
SDF forces earlier seized a nearby oilfield from IS and cut the routes from Al-Shadadi to Mosul in neighbouring Iraq as well as IS's de facto Syrian capital Raqa.
The SDF began a new operation in Hasakeh on Tuesday, following major advances by its forces in northern Aleppo province, where it has seized territory from Syrian rebel groups.
The alliance groups the powerful Kurdish People's Protection Units (YPG) and smaller Arab groups.
Its advances in Aleppo have angered Turkey, which says the YPG is the Syrian affiliate of the Kurdistan Workers' Party, an outlawed group that has waged a decades-long insurgency against Ankara.
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On Friday, Turkish President Recep Tayyip Erdogan repeated accusations that the YPG was behind a Wednesday bombing in Ankara that killed 28 people.
He said he would speak to US President Barack Obama to warn him over US weapons support to the YPG, a key ally for Washington in the fight against IS.
Turkey has carried out nearly a week of shelling against SDF positions in Aleppo, intensifying its fire Friday and expanding it to target the Kurdish town of Afrin, where two civilians were killed, the Observatory said.
Ankara fears the SDF advance in Aleppo province is intended to connect Kurdish-held areas in northern and northeastern Syria, creating an autonomous Kurdish region along much of its southern border.
The Syrian conflict, which began with anti-government protests in March 2011, has become increasingly complex, drawing in several international players.
Key regime backer Russia launched air strikes in support of President Bashar al-Assad's government in September, raising tensions with Turkey.
Ankara has in recent days raised the possibility of sending ground troops into Syria to fight IS, sparking Russian criticism.
- Peace talks resumption delayed -
Moscow said it was convening a UN Security Council meeting Friday "to discuss this issue and to introduce a draft resolution calling for a halt to any actions that would undermine the sovereignty and territorial integrity of Syria".
Saudi's foreign minister meanwhile said Syria's moderate rebels should receive surface-to-air missiles, saying they would "change the balance of power on the ground".
Speaking to the German Der Spiegel magazine, he acknowledged though that the idea should "be studied very carefully because you don't want such weapons to fall into the wrong hands".
Syria's conflict is now approaching its sixth year, with more than 260,000 people killed and half the population displaced.
World powers meeting in Munich last week announced a plan for humanitarian access throughout Syria and a ceasefire to pave the way for a return to negotiations that collapsed earlier this month.
But De Mistura told a Swedish newspaper Friday that he could not "realistically call for new Geneva talks starting on February 25".
"We need 10 days of preparations and invitations. But we will aim to do this soon," the Svenska Dagbladet newspaper quoted him as saying.
He also said he hoped Moscow and Washington could agree a "beginning of a cessation of hostilities (in Syria) between today and mid-next week."